rust: drop support for different optimization levels than C

Since the C side only supports `-O2` or higher, we can also
remove a few other bits too.

Link: https://lore.kernel.org/lkml/CAKwvOdkjttdX83tL4pw+J5EnHM1MgEYDPp=YTpEagV4RrhdxwA@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAKwvOd=7QTUH69+ZbT7e8einvgcosTbDkyohmPaUBv6_y8RfrQ@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CANiq72kySVvOQ7eqwe0Jzz3V0JTtrcqODHR9Ty4-sfDMdzP6XQ@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAKwvOdn+9qORm8UpDGnPXxiK7B7P_TW5CtXv1+8qkv7UvQr3hQ@mail.gmail.com/
Link: https://lore.kernel.org/lkml/CAKwvOdkTjxNEmCTnuH5f41WB50ef6ErBM2Kp2zJ-t9q_5U8rBA@mail.gmail.com/
Suggested-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Miguel Ojeda <ojeda@kernel.org>

Documentation/rust/arch-support.rst

@@ -15,11 +15,9 @@ support corresponds to ``S`` values in the ``MAINTAINERS`` file.
 ============ ================ ==============================================
 Architecture Level of support Constraints
 ============ ================ ==============================================
-``arm`` Maintained ``armv6`` and compatible only,
- ``RUST_OPT_LEVEL >= 2``.
+``arm`` Maintained ``armv6`` and compatible only.
 ``arm64`` Maintained None.
-``powerpc`` Maintained ``ppc64le`` only, ``RUST_OPT_LEVEL < 2``
- requires ``CONFIG_THREAD_SHIFT=15``.
+``powerpc`` Maintained ``ppc64le`` only.
 ``riscv`` Maintained ``riscv64`` only.
 ``x86`` Maintained ``x86_64`` only.
 ============ ================ ==============================================

Makefile

@@ -819,27 +819,19 @@ KBUILD_CFLAGS += $(call cc-disable-warning, address-of-packed-member)
 
 ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE
 KBUILD_CFLAGS += -O2
-KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 2
+KBUILD_RUSTFLAGS += -Copt-level=2
 else ifdef CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE_O3
 KBUILD_CFLAGS += -O3
-KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := 3
+KBUILD_RUSTFLAGS += -Copt-level=3
 else ifdef CONFIG_CC_OPTIMIZE_FOR_SIZE
 KBUILD_CFLAGS += -Os
-KBUILD_RUSTFLAGS_OPT_LEVEL_MAP := s
+KBUILD_RUSTFLAGS += -Copt-level=s
 endif
 
 # Always set `debug-assertions` and `overflow-checks` because their default
 # depends on `opt-level` and `debug-assertions`, respectively.
 KBUILD_RUSTFLAGS += -Cdebug-assertions=$(if $(CONFIG_RUST_DEBUG_ASSERTIONS),y,n)
 KBUILD_RUSTFLAGS += -Coverflow-checks=$(if $(CONFIG_RUST_OVERFLOW_CHECKS),y,n)
-KBUILD_RUSTFLAGS += -Copt-level=$\
- $(if $(CONFIG_RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C),$(KBUILD_RUSTFLAGS_OPT_LEVEL_MAP))$\
- $(if $(CONFIG_RUST_OPT_LEVEL_0),0)$\
- $(if $(CONFIG_RUST_OPT_LEVEL_1),1)$\
- $(if $(CONFIG_RUST_OPT_LEVEL_2),2)$\
- $(if $(CONFIG_RUST_OPT_LEVEL_3),3)$\
- $(if $(CONFIG_RUST_OPT_LEVEL_S),s)$\
- $(if $(CONFIG_RUST_OPT_LEVEL_Z),z)
 
 # Tell gcc to never replace conditional load with a non-conditional one
 ifdef CONFIG_CC_IS_GCC

lib/Kconfig.debug

@@ -2720,81 +2720,9 @@ config RUST_OVERFLOW_CHECKS
 
  If unsure, say Y.
 
-choice
- prompt "Optimization level"
- default RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C
- depends on RUST
- help
- Controls rustc's `-Copt-level` codegen option.
-
- This flag controls the optimization level.
-
- If unsure, say "Similar as chosen for C".
-
-config RUST_OPT_LEVEL_SIMILAR_AS_CHOSEN_FOR_C
- bool "Similar as chosen for C"
- help
- This choice will pick a similar optimization level as chosen in
- the "Compiler optimization level" for C:
-
- -O2 is currently mapped to -Copt-level=2
- -O3 is currently mapped to -Copt-level=3
- -Os is currently mapped to -Copt-level=s
-
- The mapping may change over time to follow the intended semantics
- of the choice for C as sensibly as possible.
-
- This is the default.
-
-config RUST_OPT_LEVEL_0
- bool "No optimizations (-Copt-level=0)"
- help
- Not recommended for most purposes. It may come in handy for debugging
- suspected optimizer bugs, unexpected undefined behavior, etc.
-
- Note that this level will *not* enable debug assertions nor overflow
- checks on its own (like it happens when interacting with rustc
- directly). Use the corresponding configuration options to control
- that instead, orthogonally.
-
- Note this level may cause excessive stack usage, which can lead to stack
- overflow and subsequent crashes.
-
-config RUST_OPT_LEVEL_1
- bool "Basic optimizations (-Copt-level=1)"
- help
- Useful for debugging without getting too lost, but without
- the overhead and boilerplate of no optimizations at all.
-
- Note this level may cause excessive stack usage, which can lead to stack
- overflow and subsequent crashes.
-
-config RUST_OPT_LEVEL_2
- bool "Some optimizations (-Copt-level=2)"
- help
- The sensible choice in most cases.
-
-config RUST_OPT_LEVEL_3
- bool "All optimizations (-Copt-level=3)"
- help
- Yet more performance (hopefully).
-
-config RUST_OPT_LEVEL_S
- bool "Optimize for size (-Copt-level=s)"
- help
- Smaller kernel, ideally without too much performance loss.
-
-config RUST_OPT_LEVEL_Z
- bool "Optimize for size, no loop vectorization (-Copt-level=z)"
- help
- Like the previous level, but also turn off loop vectorization.
-
-endchoice
-
 choice
  prompt "Build-time assertions"
- default RUST_BUILD_ASSERT_ALLOW if RUST_OPT_LEVEL_0
- default RUST_BUILD_ASSERT_DENY if !RUST_OPT_LEVEL_0
+ default RUST_BUILD_ASSERT_DENY
  depends on RUST
  help
  Controls how are `build_error!` and `build_assert!` handled during build.
@@ -2822,7 +2750,6 @@ config RUST_BUILD_ASSERT_WARN
 
 config RUST_BUILD_ASSERT_DENY
  bool "Deny"
- depends on !RUST_OPT_LEVEL_0
  help
  Unoptimized calls to `build_error!` will abort compilation.