Blinkenlights

This project contains two programs:

blink is a virtual machine that runs x86-64-linux programs on different operating systems and hardware architectures. It's designed to do the same thing as the qemu-x86_64 command, except that

1. Blink is 221kb in size (115kb with optional features disabled), whereas qemu-x86_64 is a 4mb binary.

2. Blink will run your Linux binaries on any POSIX system, whereas qemu-x86_64 only supports Linux.

3. Blink goes 2x faster than qemu-x86_64 on some benchmarks, such as SSE integer / floating point math. Blink is also much faster at running ephemeral programs such as compilers.

blinkenlights is a terminal user interface that may be used for debugging x86_64-linux or i8086 programs across platforms. Unlike GDB, Blinkenlights focuses on visualizing program execution. It uses UNICODE IBM Code Page 437 characters to display binary memory panels, which change as you step through your program's assembly code. These memory panels may be scrolled and zoomed using your mouse wheel. Blinkenlights also permits reverse debugging, where scroll wheeling over the assembly display allows the rewinding of execution history.

Getting Started

We regularly test that Blink is able run x86-64-linux binaries on the following platforms:

• Linux (x86, ARM, RISC-V, MIPS, PowerPC, s390x)
• macOS (x86, ARM)
• FreeBSD
• OpenBSD
• Cygwin

Blink depends on the following libraries:

• libc (POSIX.1-2017 with XSI extensions)

Blink can be compiled on UNIX systems that have:

• A C11 compiler with atomics (e.g. GCC 4.9.4+)
• Modern GNU Make (i.e. not the one that comes with XCode)

The instructions for compiling Blink are as follows:

./configure
make -j4
doas make install  # note: doas is modern sudo
blink -v
man blink

Here's how you can run a simple hello world program with Blink:

blink third_party/cosmo/tinyhello.elf

Blink has a debugger TUI, which works with UTF-8 ANSI terminals. The most important keystrokes in this interface are ? for help, s for step, c for continue, and scroll wheel for reverse debugging.

blinkenlights third_party/cosmo/tinyhello.elf

Alternative Builds

For maximum tinyness, use MODE=tiny, since it makes Blink's binary footprint 50% smaller. The Blink executable should be on the order of 200kb in size. Performance isn't impacted. Please note that all assertions will be removed, as well as all logging. Use this mode if you're confident that Blink is bug-free for your use case.

make MODE=tiny
strip o/tiny/blink/blink
ls -hal o/tiny/blink/blink

Some distros configure their compilers to add a lot of security bloat, which might add 60kb or more to the above binary size. You can work around that by using one of Blink's toolchains. This should produce consistently the smallest possible executable size.

make MODE=tiny o/tiny/x86_64/blink/blink
o/third_party/gcc/x86_64/bin/x86_64-linux-musl-strip o/tiny/x86_64/blink/blink
ls -hal o/tiny/x86_64/blink/blink

If you want to make Blink even tinier (more on the order of 120kb rather than 200kb) than you can tune the ./configure script to disable optional features such as jit, threads, sockets, x87, bcd, xsi, etc.

./configure --disable-all --posix
make MODE=tiny o/tiny/x86_64/blink/blink
o/third_party/gcc/x86_64/bin/x86_64-linux-musl-strip o/tiny/x86_64/blink/blink
ls -hal o/tiny/x86_64/blink/blink

The traditional MODE=rel or MODE=opt modes are available. Use this mode if you're on a non-JIT architecture (since this won't improve performance on AMD64 and ARM64) and you're confident that Blink is bug-free for your use case, and would rather have Blink not create a blink.log or print SIGSEGV delivery warnings to standard error, since many apps implement their own crash reporting.

make MODE=rel
o/rel/blink/blink -h

You can hunt down bugs in Blink using the following build modes:

• MODE=asan helps find memory safety bugs
• MODE=tsan helps find threading related bugs
• MODE=ubsan to find violations of the C standard
• MODE=msan helps find uninitialized memory errors

You can check Blink's compliance with the POSIX standard using the following configuration flags:

./configure --posix  # only use c11 with posix xopen standard

If you want to run a full chroot'd Linux distro and require correct handling of absolute symlinks, displaying of certain values in /proc, and so on, and you don't mind paying a small price in terms of size and performance, you can enable the emulated VFS feature by using the following configuration:

./configure --enable-vfs

Testing

Blink is tested primarily using precompiled binaries downloaded automatically. Blink has more than 700 test programs total. You can check how well Blink works on your local platform by running:

make check

To check that Blink works on 11 different hardware $(ARCHITECTURES) (see Makefile), you can run the following command, which will download statically-compiled builds of GCC and Qemu. Since our toolchain binaries are intended for x86-64 Linux, Blink will bootstrap itself locally first, so that it's possible to run these tests on other operating systems and architectures.

make check2
make emulates

Production Worthiness

Blink passes 194 test suites from the Cosmopolitan Libc project (see third_party/cosmo). Blink passes 350 test suites from the Linux Test Project (see third_party/ltp). Blink passes 108 of Musl Libc's unit test suite (see third_party/libc-test). The tests we haven't included are because either (1) it wanted x87 long double to have 80-bit precision, or (2) it used Linux APIs we can't or won't support, e.g. System V message queues. Blink runs the precompiled Linux test binaries above on other operating systems too, e.g. Apple M1, FreeBSD, Cygwin.

Reference

The Blinkenlights project provides two programs which may be launched on the command line.

blink Flags

The headless Blinkenlights virtual machine command (named blink by convention) accepts command line arguments per the specification:

blink [FLAG...] PROGRAM [ARG...]

Where PROGRAM is an x86_64-linux binary that may be specified as:

1. An absolute path to an executable file, which will be run as-is
2. A relative path containing slashes, which will be run as-is
3. A path name without slashes, which will be $PATH searched

The following FLAG arguments are provided:

• -h shows help on command usage

• -v shows version and build configuration details

• -e means log to standard error (fd 2) in addition to the log file. If logging to only standard error is desired, then -eL/dev/null may be used.

• -j disables Just-In-Time (JIT) compilation, which will make Blink go ~10x slower.

• -m disables the linear memory optimization. This makes Blink memory safe, but comes at the cost of going ~4x slower. On some platforms this can help avoid the possibility of an mmap() crisis.

• -0 allows argv[0] to be specified on the command line. Under normal circumstances, blink cmd arg1 is equivalent to execve("cmd", {"cmd", "arg1"}) since that's how most programs are launched. However if you need the full power of execve() process spawning, you can say blink -0 cmd arg0 arg1 which is equivalent to execve("cmd", {"arg0", "arg1"}).

• -L PATH specifies the log path. The default log path is blink.log in the current directory at startup. This log file won't be created until something is actually logged. If logging to a file isn't desired, then -L /dev/null may be used. See also the -e flag for logging to standard error.

• -s enables system call logging. This will emit to the log file the names of system calls each time a SYSCALL instruction in executed, along with its arguments and result. System calls are logged once they've completed. If this option is specified twice, then system calls which are likely to block (e.g. poll) will be logged at entry too. If this option is specified thrice, then all cancellation points will be logged upon entry. System call logging isn't available in MODE=rel and MODE=tiny builds, in which case this flag is ignored.

• -Z will cause internal statistics to be printed to standard error on exit. Stats aren't available in MODE=rel and MODE=tiny builds, and this flag is ignored.

• -C path will cause blink to launch the program in a chroot'd environment. This flag is both equivalent to and overrides the BLINK_OVERLAYS environment variable. Note: This flag works especially well if you use ./configure --enable-vfs.

blinkenlights Flags

The Blinkenlights ANSI TUI interface command (named blinkenlights by convention) accepts its command line arguments in accordance with the following specification:

blinkenlights [FLAG...] PROGRAM [ARG...]

Where PROGRAM is an x86_64-linux binary that may be specified as:

1. An absolute path to an executable file, which will be run as-is
2. A relative path containing slashes, which will be run as-is
3. A path name without slashes, which will be $PATH searched

The following FLAG arguments are provided:

• -h shows help on command usage

• -v shows version and build configuration details

• -r puts your virtual machine in real mode. This may be used to run 16-bit i8086 programs, such as SectorLISP. It's also used for booting programs from Blinkenlights's simulated BIOS.

• -b ADDR pushes a breakpoint, which may be specified as a raw hexadecimal address, or a symbolic name that's defined by your ELF binary (or its associated .dbg file). When pressing c (continue) or C (continue harder) in the TUI, Blink will immediately stop upon reaching an instruction that's listed as a breakpoint, after which a modal dialog is displayed. The modal dialog may be cleared by ENTER after which the TUI resumes its normal state.

• -w ADDR pushes a watchpoint, which may be specified as a raw hexadecimal address, or a symbolic name that's defined by your ELF binary (or its associated .dbg file). When pressing c (continue) or C (continue harder) in the TUI, Blink will immediately stop upon reaching an instruction that either (a) has a ModR/M encoding that references an address that's listed as a watchpoint, or (b) manages to mutate the memory stored at a watchpoint address by some other means. When Blinkenlights is stopped at a watchpoint, a modal dialog will be displayed which may be cleared by pressing ENTER, after which the TUI resumes its normal state.

• -j enables Just-In-Time (JIT) compilation. This will make Blinkenlights go significantly faster, at the cost of taking away the ability to step through each instruction. The TUI will visualize JIT path formation in the assembly display; see the JIT Path Glyphs section below to learn more. Please note this flag has the opposite meaning as it does in the blink command.

• -m enables the linear memory optimization. This makes blinkenlights capable of faster emulation, at the cost of losing some statistics. It no longer becomes possible to display which percentage of a memory map has been activated. Blinkenlights will also remove the commit / reserve / free page statistics from the status panel on the bottom right of the display. Please note this flag has the opposite meaning as it does in the blink command.

• -t may be used to disable Blinkenlights TUI mode. This makes the program behave similarly to the blink command, however not as good. We're currently using this flag for unit testing real mode programs, which are encouraged to use the SYSCALL instruction to report their exit status.

• -L PATH specifies the log path. The default log path is $TMPDIR/blink.log or /tmp/blink.log if $TMPDIR isn't defined.

• -C path will cause blink to launch the program in a chroot'd environment. This flag is both equivalent to and overrides the BLINK_OVERLAYS environment variable.

• -s enables system call logging. This will emit to the log file the names of system calls each time a SYSCALL instruction in executed, along with its arguments and result. System calls are logged once they've completed. If this option is specified twice, then system calls which are likely to block (e.g. poll) will be logged at entry too. If this option is specified thrice, then all cancellation points will be logged upon entry. System call logging isn't available in MODE=rel and MODE=tiny builds, in which case this flag is ignored.

• -Z will cause internal statistics to be printed to standard error on exit. Each line will display a monitoring metric. Most metrics will either be integer counters or floating point running averages. Most but not all integer counters are monotonic. In the interest of not negatively impacting Blink's performance, statistics are computed on a best effort basis which currently isn't guaranteed to be atomic in a multi-threaded environment. Stats aren't available in MODE=rel and MODE=tiny builds, and this flag is ignored.

• -z [repeatable] may be specified to zoom the memory panels, so they display a larger amount of memory in a smaller space. By default, one terminal cell corresponds to a single byte of memory. When memory has been zoomed the magic kernel is used (similar to Lanczos) to decimate the number of bytes by half, for each -z that's specified. Normally this would be accomplished by using CTRL+MOUSEWHEEL where the mouse cursor is hovered over the panel that should be zoomed. However, many terminal emulators (especially on Windows), do not support this xterm feature and as such, this flag is provided as an alternative.

• -V [repeatable] increases verbosity

• -R disables reactive error mode

• -H disables syntax highlighting

• -N enables natural scrolling

Recommended Environments

Blinkenlights' TUI requires a UTF-8 VT100 / XTERM style terminal to use. We recommend the following terminals, ordered by preference:

• KiTTY (Linux)
• PuTTY (Windows)
• Gnome Terminal (Linux)
• Terminal.app (macOS)
• CMD.EXE (Windows 10+)
• PowerShell (Windows 10+)
• Xterm (Linux)

The following fonts are recommended, ordered by preference:

• PragmataPro Regular Mono (€59)
• Bitstream Vera Sans Mono (a.k.a. DejaVu Sans Mono)
• Consolas
• Menlo

JIT Path Glyphs

When the Blinkenlights TUI is run with JITing enabled (using the -j flag) the assembly dump display will display a glyph next to the address of each instruction, to indicate the status of JIT path formation. Those glyphs are defined as follows:

• or space indicates no JIT path is associated with an address

• S means that a JIT path is currently being constructed which starts at this address. By continuing to press s (step) in the TUI interface, the JIT path will grow longer until it is eventually completed, and the S glyph is replaced by *.

• * (asterisk) means that a JIT path has been installed to the adjacent address. When s (step) is pressed at such addresses within the TUI display, stepping takes on a different meaning. Rather than stepping a single instruction, it will step the entire length of the JIT path. The next assembly line that'll be highlighted will be the instruction after where the path ends.

Environment Variables

The following environment variables are recognized by both the blink and blinkenlights commands:

• BLINK_LOG_FILENAME may be specified to supply a log path to be used in cases where the -L PATH flag isn't specified. This value should be an absolute path. If logging to standard error is desired, use the blink -e flag.

• BLINK_OVERLAYS specifies one or more directories to use as the root filesystem. Similar to $PATH this is a colon delimited list of pathnames. If relative paths are specified, they'll be resolved to an absolute path at startup time. Overlays only apply to IO system calls that specify an absolute path. The empty string overlay means use the normal / root filesystem. The default value is :o, which means if the absolute path /$f is opened, then first check if /$f exists, and if it doesn't, then check if o/$f exists, in which case open that instead. Blink uses this convention to open shared object tests. It favors the system version if it exists, but also downloads ld-musl-x86_64.so.1 to o/lib/ld-musl-x86_64.so.1 so the dynamic linker can transparently find it on platforms like Apple, that don't let users put files in the root folder. On the other hand, it's possible to say BLINK_OVERLAYS=o: so that o/... takes precedence over /... (noting again that empty string means root). If a single overlay is specified that isn't empty string, then it'll effectively act as a restricted chroot environment.

Compiling and Running Programs under Blink

Blink can be picky about which Linux binaries it'll execute. It may also be the case that your Linux binary will only run under Blink on Linux, but report errors if run under Blink on another platform, e.g. macOS. In our experience, how successfully a program can run under Blink depends almost entirely on (1) how it was compiled, and (2) which C library it uses. This section will provide guidance on which tools will work best.

First, some background. Blink's coverage of the x86_64 instruction set is comprehensive. However the Linux system call ABI is much larger and therefore not possible to fully support, unless Blink emulated a Linux kernel image too. Blink has sought to support the subset of Linux ABIs that are either (1) standardized by POSIX.1-2017 or (2) too popular to not support. As an example, AF_INET, AF_UNIX, and AF_INET6 are supported, but Blink will return EINVAL if a program requests any of the dozens of other ones, e.g. AF_BLUETOOTH. Such errors are usually logged to /tmp/blink.log, to make it easy to file a feature request. In other cases ABIs aren't supported simply because they're Linux-only and difficult to polyfill on other POSIX platforms. For example, Blink will polyfill open(O_TMPFILE) on non-Linux platforms so it works the same way, but other Linux-specific ABIs like membarrier() we haven't had the time to figure out yet. Since app developers usually don't use non-portable APIs, it's usually the platform C library that's at fault for calling them. Many Linux system calls, could be rightfully thought of as an implementation detail of Glibc.

Blink's prime directive is to support binaries built with Cosmopolitan Libc. Actually Portable Executables make up the bulk of Blink's unit test suite. Anything created by Cosmopolitan is almost certainly going to work very well. Since Cosmopolitan is closely related to Musl Libc, programs compiled using Musl also tend to work very well. For example, Alpine Linux is a Musl Libc based distro, so their prebuilt dynamic binaries tend to all work well, and it's also a great platform to use for compiling other software from source that's intended for Blink.

So the recommended approach is either:

1. Build your app using Cosmopolitan Libc, otherwise
2. Build your app using GNU Autotools on Alpine Linux
3. Build your app using Buildroot

For Cosmopolitan, please read Getting Started with Cosmopolitan Libc for information on how to get started. Cosmopolitan comes with a lot of third party software included that you can try with Blink right away, e.g. SQLite, Python, QuickJS, and Antirez's Kilo editor.

git clone https://github.com/jart/cosmopolitan/
cd cosmopolitan

make -j8 o//third_party/python/python.com
blinkenlights -jm o//third_party/python/python.com

make -j8 o//third_party/quickjs/qjs.com
blinkenlights -jm o//third_party/quickjs/qjs.com

make -j8 o//third_party/sqlite3/sqlite3.com
blinkenlights -jm o//third_party/sqlite3/sqlite3.com

make -j8 o//examples/kilo.com
blinkenlights -jm o//examples/kilo.com

Blink is great for making single-file autonomous binaries like the above easily copyable across platforms. If you're more interested in building systems instead, then Buildroot is one way to create a Linux userspace that'll run under Blink. All you have to do is set the $BLINK_OVERLAYS environment variable to the buildroot target folder, which will ask Blink to create a chroot'd environment.

cd ~/buildroot
export CC="gcc -Wl,-z,common-page-size=65536,-z,max-page-size=65536"
make menuconfig
make
cp -R output/target ~/blinkroot
doas mount -t devtmpfs none ~/blinkroot/dev
doas mount -t sysfs none ~/blinkroot/sys
doas mount -t proc none ~/blinkroot/proc
cd ~/blink
make -j8
export BLINK_OVERLAYS=$HOME/blinkroot
blink sh
uname -a
Linux hostname 4.5 blink-1.0 x86_64 GNU/Linux

If you want to build an Autotools project like Emacs, the best way to do that is to spin up an Alpine Linux container and use jart/blink-isystem as your system header subset. blink-isystem is basically just the Musl Linux headers with all the problematic APIs commented out. That way autoconf won't think the APIs Blink doesn't have are available, and will instead configure Emacs to use portable alternatives. Setting this up is simple:

./configure CFLAGS="-isystem $HOME/blink-isystem" \
            CXXFLAGS="-isystem $HOME/blink-isystem" \
            LDFLAGS="-static -Wl,-z,common-page-size=65536,-z,max-page-size=65536"
make -j

Another big issue is the host system page size may cause problems on non-Linux platforms like Apple M1 (16kb) and Cygwin (64kb). On such platforms, you may encounter an error like this:

p_vaddr p_offset skew unequal w.r.t. host page size

The simplest way to solve that is by disabling the linear memory optimization (using the blink -m flag) but that'll slow down performance. Another option is to try recompiling your executable so that its ELF program headers will work on systems with a larger page size. You can do that using these GCC flags:

gcc -static -Wl,-z,common-page-size=65536,-z,max-page-size=65536 ...

However that's just step one. The program also needs to be using APIs like sysconf(_SC_PAGESIZE) which will return the true host page size, rather than naively assuming it's 4096 bytes. Your C library gets this information from Blink via getauxval(AT_PAGESZ).

If you're using the Blinkenlights debugger TUI, then another important set of flags to use are the following:

• -fno-omit-frame-pointer
• -mno-omit-leaf-frame-pointer

By default, GCC and Clang use the %rbp backtrace pointer as a general purpose register, and as such, Blinkenlights won't be able to display a frames panel visualizing your call stack. Using those flags solves that. However it's tricky sometimes to correctly specify them in a complex build environment, where other optimization flags might subsequently turn them back off again.

The trick we recommend using for compiling your programs, is to create a shell script that wraps your compiler command, and then use the script in your $CC environment variable. The script should look something like the following:

#!/bin/sh
set /usr/bin/gcc "$@" -g \
    -fno-omit-frame-pointer \
    -fno-optimize-sibling-calls \
    -mno-omit-leaf-frame-pointer \
    -Wl,-z,norelro \
    -Wl,-z,noseparate-code \
    -Wl,-z,max-page-size=65536 \
    -Wl,-z,common-page-size=65536
printf '%s\n' "$*" >>/tmp/gcc.log
exec "$@"

Those flags will go a long way towards helping your Linux binaries be (1) capable of running under Blink on all of its supported operating systems and microprocessor architectures, and (2) trading away some of the modern security blankets in the interest of making the assembly panel more readable, and less likely to be picky about memory.

If you're a Cosmopolitan Libc user, then Cosmopolitan already provides such a script, which is the cosmocc and cosmoc++ toolchain. Please note that Cosmopolitan Libc uses a 64kb page size so it isn't impacted by many of these issues that Glibc and Musl users may experience.

• cosmopolitan/tool/scripts/cosmocc
• cosmopolitan/tool/scripts/cosmoc++

If you're not a C / C++ developer, and you prefer to use high-level languages instead, then one program you might consider emulating is Actually Portable Python, which is an APE build of the CPython v3.6 interpreter. It can be built from source, and then used as follows:

git clone https://github.com/jart/cosmopolitan/
cd cosmopolitan
make -j8 o//third_party/python/python.com
blinkenlights -jm o//third_party/python/python.com

The -jm flags are helpful here, since they ask the Blinkenlights TUI to enable JIT and the linear memory optimization. It's helpful to have those flags because Python is a very complicated and compute intensive program, that would otherwise move too slowly under the Blinkenlights vizualization. You may also want to press the CTRL-T (TURBO) key a few times, to make Python emulate in the TUI even faster.

Technical Details

blink is an x86-64 interpreter for POSIX platforms that's written in ANSI C11 that's compatible with C++ compilers. Instruction decoding is done using our trimmed-down version of Intel's disassembler Xed.

The prime directive of this project is to act as a virtual machine for userspace binaries compiled by Cosmopolitan Libc. However we've also had success virtualizing programs compiled with Glibc and Musl Libc, such as GCC and Qemu. Blink supports 500+ instructions and 150+ Linux syscalls, including fork() and clone(). Linux system calls may only be used by long mode programs via the SYSCALL instruction, as it is written in the System V ABI.

Instruction Sets

The following hardware ISAs are supported by Blink.

• i8086
• i386
• X87
• SSE2
• x86_64
• SSE3
• SSSE3
• CLMUL
• POPCNT
• ADX
• BMI2
• RDRND
• RDSEED
• RDTSCP

Programs may use CPUID to confirm the presence or absence of optional instruction sets. Please note that Blink does not follow the same monotonic progress as Intel's hardware. For example, BMI2 is supported; this is an AVX2-encoded (VEX) instruction set, which Blink is able to decode, even though the AVX2 ISA isn't supported. Therefore it's important to not glob ISAs into "levels" (as Windows software tends to do) where it's assumed that BMI2 support implies AVX2 support; because with Blink that currently isn't the case.

On the other hand, Blink does share Windows' x87 behavior w.r.t. double (rather than long double) precision. It's not possible to use 80-bit floating point precision with Blink, because Blink simply passes along floating point operations to the host architecture, and very few architectures support long double precision. You can still use x87 with 80-bit words. Blink will just store 64-bit floating point values inside them, and that's a legal configuration according to the x87 FPU control word. If possible, it's recommended that long double simply be avoided. If 64-bit floating point is good enough for the rocket scientists at NASA then it should be good enough for everybody. There are some peculiar differences in behavior with double across architectures (which Blink currently does nothing to address) but they tend to be comparatively minor, e.g. an op returning NAN instead of -NAN.

Blink has reasonably comprehensive coverage of the baseline ISAs, including even support for BCD operations (even in long mode!). But there are some truly fringe instructions Blink hasn't implemented, such as BOUND and ENTER. Most of the unsupported instructions, are usually ring-0 system instructions, since Blink is primarily a user-mode VM, and therefore only has limited support for bare metal operating system software (which we'll discuss more in-depth in a later section).

Blink advertises itself as Linux 4.5 in the uname() system call and uname -v will report blink-1.0. Programs may detect they're running in Blink by issuing a CPUID instruction where EAX is set to the leaf number:

• Leaf 0x0 (or 0x80000000) reports GenuineIntel in EBX ‖ EDX ‖ ECX

• Leaf 0x1 reports that Blink is a hypervisor in bit 31 of ECX

• Leaf 0x40000000 reports GenuineBlink as the hypervisor name in EBX ‖ ECX ‖ EDX

• Leaf 0x40031337 reports the underlying operating system name in EBX ‖ ECX ‖ EDX with zero filling for strings shorter than 12:

  • Linux for Linux
  • XNU for macOS
  • FreeBSD for FreeBSD
  • NetBSD for NetBSD
  • OpenBSD for OpenBSD
  • Linux for Linux
  • Cygwin for Windows under Cygwin
  • Windows for Windows under Cosmopolitan
  • Unknown if compiled on unrecognized platform

• Leaf 0x80000001 tells if Blink's JIT is enabled in bit 31 in ECX

JIT

Blink uses just-in-time compilation, which is supported on x86_64 and aarch64. Blink takes the appropriate steps to work around restrictions relating to JIT, on platforms like Apple and OpenBSD. We generate JIT code using a printf-style domain-specific language. The JIT works by generating functions at runtime which call the micro-op functions the compiler created. To make micro-operations go faster, Blink determines the byte length of the compiled function at runtime by scanning for a RET instruction. Blink will then copy the compiled function into the function that the JIT is generating. This works in most cases, however some tools can cause problems. For example, OpenBSD RetGuard inserts static memory relocations into every compiled function, which Blink's JIT currently doesn't understand; so we need to use compiler flags to disable that type of magic. In the event other such magic slips through, Blink has a runtime check which will catch obvious problems, and then gracefully fall back to using a CALL instruction. Since no JIT can be fully perfect on all platforms, the blink -j flag may be passed to disable Blink's JIT. Please note that disabling JIT makes Blink go 10x slower. With the blinkenlights command, the -j flag takes on the opposite meaning, where it instead enables JIT. This can be useful for troubleshooting the JIT, because the TUI display has a feature that lets JIT path formation be visualized. Blink currently only enables the JIT for programs running in long mode (64-bit) but we may support JITing 16-bit programs in the future.

Virtualization

Blink virtualizes memory using the same PML4T approach as the hardware itself, where memory lookups are indirected through a four-level radix tree. Since performing four separate page table lookups on every memory access can be slow, Blink checks a translation lookaside buffer, which contains the sixteen most recently used page table entries. The PML4T allows all memory lookups in Blink to be "safe" but it still doesn't offer the best possible performance. Therefore, on systems with a huge address space (i.e. petabytes of virtual memory) Blink relies on itself being loaded to a random location, and then identity maps guest memory using a simple linear translation. For example, if the guest virtual address is 0x400000 then the host address might be 0x400000+0x088800000000. This means that each time a memory operation is executed, only a simple addition needs to be performed. This goes extremely fast, however it may present issues for programs that use MAP_FIXED. Some systems, such as modern Raspberry Pi, actually have a larger address space than x86-64, which lets Blink offer the guest the complete address space. However on some platforms, like 32-bit ones, only a limited number of identity mappings are possible. There's also compiler tools like TSAN which lay claim to much of the fixed address space. Blink's solution is designed to meet the needs of Cosmopolitan Libc, while working around Apple's restriction on 32-bit addresses, and still remain fully compatible with ASAN's restrictions. In the event that this translation scheme doesn't work on your system, the blink -m flag may be passed to disable the linear translation optimization, and instead use only the memory safe full virtualization approach of the PML4T and TLB.

Lockless Hashing

Blink stores generated functions by virtual address in a multithreaded lockless hash table. The hottest operation in the codebase is reading from this hash table, using a function called GetJitHook. Since it'd slow Blink down by more than 33% if a mutex were used here, Blink will synchronize reads optimistically using only carefully ordered load instructions, three of which have acquire semantics. This hash table starts off at a reasonable size and grows gradually with the memory requirements. This design is the primary reason Blink usually uses 40% less peak resident memory than Qemu.

Acyclic Codegen

Even though JIT paths will always end at branching instructions, Blink will generate code so that paths tail call into each other, in order to avoid dropping back into the main interpreter loop. The average length of a JIT path is about ~5 opcodes. Connecting paths causes the average path length to be ~13 opcodes.

Since Blink only checks for asynchronous signal delivery and shutdown events from the main interpreter loop, Blink maintains a bidirectional map of edges between generated functions, so that path connections which would result in cycles are never introduced.

An exception is made for tight conditional branches, i.e. jumps whose taken path jump backwards to the start of the JIT path. Such branches are allowed to be self-referencing so that whole loops of non-system operations may be run in purely native code.

Reliable Memory

Blink has a 22mb global static variable that's set aside for JIT code. This limit was chosen because that's roughly the maximum displacement permitted on Arm64 architecture. Having that memory near the program image helps make Blink simpler, since generated functions call normal functions, without needing relocations or procedure linkage tables.

When Blink runs out of JIT memory, it simply clears all JIT hooks and lets the whole code generation process start again. Blink is very fast at generating code, and it wouldn't make sense during an OOM panic to arbitrarily choose a subset of pages to reset, since resetting pages requires tracing their dependencies and resetting those too. Starting over is much better. It's so much better in fact, that even if Blink only reserved less than a megabyte of memory for JIT, then the slowdown that'd be incurred running 40mb binaries like GCC CC1 would only be 3x.

Blink triggers the OOM panic when only 10% of its JIT memory remains. That's because in multi-threaded programs, there's no way to guarantee nothing is still executing on the retired code blocks. Blink solves this by letting retired blocks cool off at the back of a freelist queue, so the acyclicity invariant has abundant time to ensure threads drop out.

Self-Modifying Code

Many CPU architectures require esoteric rituals for flushing CPU caches when code modifies itself. That's not the case with x86 architecture, which takes care of this chore automatically. Blink is able to offer the same promises here as Intel and AMD, by abstracting fast and automatic invalidation of caches for programs using self-modifying code (SMC).

When Blink's JIT isn't enabled, self-modifying code always causes instruction caches to be invalidated immediately, at least within the same thread. That's because Blink compares the raw instruction bytes with what's in the instruction cache before fetching its decoded value.

When JITing is enabled, Blink will automatically invalidate JIT memory associated with code that's been modified. This happens on a 4096-byte page granularity. When a function like mprotect() is called that causes memory pages to transition from a non-executable to executable state, the impacted pages will be invalidated by the JIT. The JIT maintains a hash table where the key is the virtual address at which a generated function begins (which we call a "path") and the value is a function pointer to the generated code. When Blink is generating paths, it is careful to ensure that all the guest instructions which are added to a path, only exist within the confines of a single 4096-byte page. Thus when a page needs to be invalidated, Blink simply deletes any hook for each address within the page.

When RWX memory is used, Blink can't rely on mprotect() to communicate the intent of the guest program. What Blink will do instead is protect any RWX guest memory, so that it's registered as read-only in the host operating system. This way, whenever the guest writes to RWX memory, a SIGSEGV signal will be delivered to Blink, which then re-enables write permissions on the impacted RWX page, flips a bit to the thread in the SMC state and then permits execution to resume for at least one opcode before the interpreter loop notices the SMC state, invalidates the JIT and re-enables the memory protection. This means that:

1. Memory ops in general aren't slowed down by Blink's SMC support
2. RWX memory can be written-to with some overhead
3. RWX memory can be read-from with zero overhead
4. Changes take effect when a JIT path ends

Intel's sixteen thousand page manual lays out the following guidelines for conformant self-modifying code:

To write self-modifying code and ensure that it is compliant with current and future versions of the IA-32 architectures, use one of the following coding options:

(* OPTION 1 *)
Store modified code (as data) into code segment;
Jump to new code or an intermediate location;
Execute new code;

(* OPTION 2 )
Store modified code (as data) into code segment;
Execute a serializing instruction; ( For example, CPUID instruction *)
Execute new code;

──Quoth Intel Manual V.3, §8.1.3

Blink implements this behavior because branching instructions cause JIT paths to end, paths only jump into one another selectively , and lastly serializing instructions are never added to paths in the first place.

Intel's rules allow Blink some leeway to make writing to RWX memory go fast, without causing any signal storms, or incurring too much system call overhead. As an example, consider the internal statistics printed by the smc2_test.c program:

make -j8 o//blink/blink o//test/func/smc2_test.elf
o//blink/blink -Z o//test/func/smc2_test.elf
[...]
icache_resets                    = 1
jit_blocks                       = 1
jit_hooks_installed              = 132
jit_hooks_deleted                = 19
jit_page_resets                  = 21
smc_checks                       = 22
smc_flushes                      = 22
smc_enqueued                     = 22
smc_segfaults                    = 22
[...]

The above program performs 300+ independent write operations to RWX memory. However we can see very few of them resulted in segfaults, since most of those ops happened in the SlowMemCpy() function which uses a tight conditional branch loop rather than a proper jump. This let the program get more accomplished, before dropping out of JIT code back into the main interpreter loop, which is where Blink checks the SMC state in order to flush the caches reapply any missing write protection.

Pseudoteletypewriter

Blink has an xterm-compatible ANSI pseudoteletypewriter display implementation which allows Blink's TUI interface to host other TUI programs, within an embedded terminal display. For example, it's possible to use Antirez's Kilo text editor inside Blink's TUI. For the complete list of ANSI sequences which are supported, please refer to blink/pty.c.

In real mode, Blink's PTY can be configured via INT $0x16 to convert CGA memory stored at address 0xb0000 into UNICODE block characters, thereby making retro video gaming in the terminal possible.

Real Mode

Blink supports 16-bit BIOS programs, such as SectorLISP. To boot real mode programs in Blink, the blinkenlights -r flag may be passed, which puts the virtual machine in i8086 mode. Currently only a limited set of BIOS APIs are available. For example, Blink supports IBM PC Serial UART, CGA, and MDA. We hope to expand our real mode support in the near future, in order to run operating systems like ELKS.

Blink supports troubleshooting operating system bootloaders. Blink was designed for Cosmopolitan Libc, which embeds an operating system in each binary it compiles. Blink has helped us debug our bare metal support, since Blink is capable of running in the 16-bit, 32-bit, and 64-bit modes a bootloader requires at various stages. In order to do that, we needed to implement some ring0 hardware instructions. Blink has enough to support Cosmopolitan, but it'll take much more time to get Blink to a point where it can boot something like Windows.

Executable Formats

Blink supports several different executable formats. You can run:

• x86-64-linux ELF executables (both static and dynamic).

• Actually Portable Executables, which have either the MZqFpD or jartsr magic.

• Flat executables, which must end with the file extension .bin. In this case, you can make executables as small as 10 bytes in size, since they're treated as raw x86-64 code. Blink always loads flat executables to the address 0x400000 and automatically appends 16mb of BSS memory.

• Real mode executables, which are loaded to the address 0x7c00. These programs must be run using the blinkenlights command with the -r flag.

Filesystems

When Blink is built with the VFS feature enabled (--enable-vfs), it comes with three default filesystems:

• hostfs: A filesystem that mirrors a certain directory on the host's filesystem. Files on hostfs mounts have everything read from and written directly to the corresponding host directory, with the exception of st_dev and st_ino fields. st_dev is managed by Blink's VFS subsystem, while st_ino is calculated using a hash function based on the host's st_dev and st_ino value.
• proc: A filesystem that emulates Linux's /proc using information available to Blink.
• devfs: A filesystem that emulates Linux's /dev. Currently, this is only a wrapper for hostfs.

When Blink is launched, these default mount points are added:

• / of type hostfs pointing to the corresponding host directory. This is determined by querying $BLINK_PREFIX and the -C parameter in order and falls back to / if neither are available.
• /proc of type proc.
• /dev of type devfs.
• /SytemRoot of type hostfs pointing to the host's root /.

It is possbile for programs to add additional mount points by using the mount syscall (for hostfs mounts, pass the path to the directory on the host as the source argument), but see the quirks below.

Quirks

Here's the current list of Blink's known quirks and tradeoffs.

Flags

Flag dependencies may not carry across function call boundaries under long mode. This is because when Blink's JIT is speculating whether or not it's necessary for an arithmetic instruction to compute flags, it considers RET and CALL terminal ops that break the chain. As such 64-bit code shouldn't do things we did in the DOS days, such as using carry flag as a return value to indicate error. This should work fine when STC is used to set the carry flag, but if the code computes it cleverly using instructions like SUB, then EFLAGS might not change.

Faults

Blink may not report the precise program counter where a fault occurred in ucontext_t::uc_mcontext::rip when signalling a segmentation fault. This is currently only possible when PUSH or POP access bad memory. That's because Blink's JIT tries to avoid updating Machine::ip on ops it considers "pure" such as those that only access registers, which for reasons of performance is defined to include pushing and popping.

Threads

Blink doesn't have a working implementation of set_robust_list() yet, which means robust mutexes might not get unlocked if a process crashes.

Coherency

POSIX.1 provides almost no guarantees of coherency, synchronization, and durability when it comes to MAP_SHARED mappings and recommends that msync() be explicitly used to synchronize memory with file contents. The Linux Kernel implements shared memory so well, that this is rarely necessary. However some platforms like OpenBSD lack write coherency. This means if you change a shared writable memory map and then call pread() on the associated file region, you might get stale data. Blink isn't able to polyfill incoherent platforms to be as coherent as Linux, therefore apps that run in Blink should assume the POSIX rules apply.

Signal Handling

Blink uses SIGSYS to deliver signals internally. This signal is precious to Blink. It's currently not possible for guest applications to capture it from external processes.

Memory Protection

Blink offers guest programs a 48-bit virtual address space with a 4096-byte page size. When programs are run on (1) host systems that have a larger page (e.g. Apple M1, Cygwin), and (2) the linear memory optimization is enabled (i.e. you're not using blink -m) then Blink may need to relax memory protections in cases where the memory intervals defined by the guest aren't aligned to the host system page size. This means that, on system with a larger than 4096 byte page size:

1. Misaligned read-only pages could become writable
2. JIT hooks might not invalidate automatically on misaligned RWX pages

It's recommended, when calling functions like mmap() and mprotect(), that both addr and addr + size be aliged to the host page size. Blink reports that value to the guest program in getauxval(AT_PAGESZ), which should be obtainable via the POSIX API sysconf(_SC_PAGESIZE) if the C library is implemented correctly. Please note that when Blink is running in full virtualization mode (i.e. blink -m) this concern no longer applies. That's because Blink will allocate a full system page for every 4096 byte page that's mapped from a file.

Process Management

For builds with the VFS feature enabled (--enable-vfs), while a procfs mount is available at /proc, it is limited to information available in a single process. Only /proc/self and the corresponding PID folder is available. This means programs can get the expected values at /proc/self/exe and similar files, but process management tools like ps will not work.

On Linux, some procfs symlinks possess a hardlink-like ability of being dereferenceable even after the target has been unlinked. Blink's implementation currently does not support this use case.

Mounts

For builds with the VFS feature enabled (--enable-vfs), Blink does not share mount information with other emulated processes. As a result, commands like this may seem to work (by return a 0 status code):

mount -t hostfs /some/path/on/host folder

But subsequent calls to ls folder on the same shell still does not display the expected contents. This is because the mount command could only modify the mount table kept by itself (and propagated to children through fork), but not the one used by its parent shell. In other words, Blink behaves as if CLONE_NEWNS is added to every clone call, separating the mount namespace of the child from its parent.

Some might view this behavior as a feature, but it diverges from classic system behavior; a mechanism for handling shared process state is being considered in #92.