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ARM: vfp: Reimplement VFP exception entry in C code
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En/disabling softirqs from asm code turned out to be trickier than
expected, so vfp_support_entry now returns by tail calling
__local_enable_bh_ip() and passing the same arguments that a C call to
local_bh_enable() would pass. However, this is slightly hacky, as we
don't want to carry our own implementation of local_bh_enable().

So let's bite the bullet, and get rid of the asm logic in
vfp_support_entry that reasons about whether or not to save and/or
reload the VFP state, and about whether or not an FP exception is
pending, and only keep the VFP loading logic as a function that is
callable from C.

Replicate the removed logic in vfp_entry(), and use the exact same
reasoning as in the asm code. To emphasize the correspondence, retain
some of the asm comments in the C version as well.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
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ardbiesheuvel committed May 17, 2023
1 parent 4a0548c commit 4708fb0
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Showing 4 changed files with 124 additions and 216 deletions.
12 changes: 6 additions & 6 deletions arch/arm/vfp/entry.S
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Expand Up @@ -22,10 +22,10 @@
@ IRQs enabled.
@
ENTRY(do_vfp)
mov r1, r10
str lr, [sp, #-8]!
add r3, sp, #4
str r9, [r3]
bl vfp_entry
ldr pc, [sp], #8
mov r1, r0 @ pass trigger opcode via R1
mov r0, sp @ pass struct pt_regs via R0
bl vfp_support_entry @ dispatch the VFP exception
cmp r0, #0 @ handled successfully?
reteq r9 @ then use R9 as return address
ret lr @ pass to undef handler
ENDPROC(do_vfp)
1 change: 1 addition & 0 deletions arch/arm/vfp/vfp.h
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Expand Up @@ -375,3 +375,4 @@ struct op {
};

asmlinkage void vfp_save_state(void *location, u32 fpexc);
asmlinkage u32 vfp_load_state(const void *location);
204 changes: 14 additions & 190 deletions arch/arm/vfp/vfphw.S
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Expand Up @@ -4,12 +4,6 @@
*
* Copyright (C) 2004 ARM Limited.
* Written by Deep Blue Solutions Limited.
*
* This code is called from the kernel's undefined instruction trap.
* r1 holds the thread_info pointer
* r3 holds the return address for successful handling.
* lr holds the return address for unrecognised instructions.
* sp points to a struct pt_regs (as defined in include/asm/proc/ptrace.h)
*/
#include <linux/init.h>
#include <linux/linkage.h>
Expand All @@ -19,20 +13,6 @@
#include <asm/assembler.h>
#include <asm/asm-offsets.h>

.macro DBGSTR, str
#ifdef DEBUG
stmfd sp!, {r0-r3, ip, lr}
ldr r0, =1f
bl _printk
ldmfd sp!, {r0-r3, ip, lr}

.pushsection .rodata, "a"
1: .ascii KERN_DEBUG "VFP: \str\n"
.byte 0
.previous
#endif
.endm

.macro DBGSTR1, str, arg
#ifdef DEBUG
stmfd sp!, {r0-r3, ip, lr}
Expand All @@ -48,177 +28,25 @@
#endif
.endm

.macro DBGSTR3, str, arg1, arg2, arg3
#ifdef DEBUG
stmfd sp!, {r0-r3, ip, lr}
mov r3, \arg3
mov r2, \arg2
mov r1, \arg1
ldr r0, =1f
bl _printk
ldmfd sp!, {r0-r3, ip, lr}

.pushsection .rodata, "a"
1: .ascii KERN_DEBUG "VFP: \str\n"
.byte 0
.previous
#endif
.endm


@ VFP hardware support entry point.
@
@ r0 = instruction opcode (32-bit ARM or two 16-bit Thumb)
@ r1 = thread_info pointer
@ r2 = PC value to resume execution after successful emulation
@ r3 = normal "successful" return address
@ lr = unrecognised instruction return address
@ IRQs enabled.
ENTRY(vfp_support_entry)
ldr r11, [r1, #TI_CPU] @ CPU number
add r10, r1, #TI_VFPSTATE @ r10 = workspace

DBGSTR3 "instr %08x pc %08x state %p", r0, r2, r10

.fpu vfpv2
VFPFMRX r1, FPEXC @ Is the VFP enabled?
DBGSTR1 "fpexc %08x", r1
tst r1, #FPEXC_EN
bne look_for_VFP_exceptions @ VFP is already enabled

DBGSTR1 "enable %x", r10
ldr r9, vfp_current_hw_state_address
orr r1, r1, #FPEXC_EN @ user FPEXC has the enable bit set
ldr r4, [r9, r11, lsl #2] @ vfp_current_hw_state pointer
bic r5, r1, #FPEXC_EX @ make sure exceptions are disabled
cmp r4, r10 @ this thread owns the hw context?
#ifndef CONFIG_SMP
@ For UP, checking that this thread owns the hw context is
@ sufficient to determine that the hardware state is valid.
beq vfp_hw_state_valid

@ On UP, we lazily save the VFP context. As a different
@ thread wants ownership of the VFP hardware, save the old
@ state if there was a previous (valid) owner.

VFPFMXR FPEXC, r5 @ enable VFP, disable any pending
@ exceptions, so we can get at the
@ rest of it

DBGSTR1 "save old state %p", r4
cmp r4, #0 @ if the vfp_current_hw_state is NULL
beq vfp_reload_hw @ then the hw state needs reloading
VFPFSTMIA r4, r5 @ save the working registers
VFPFMRX r5, FPSCR @ current status
tst r1, #FPEXC_EX @ is there additional state to save?
beq 1f
VFPFMRX r6, FPINST @ FPINST (only if FPEXC.EX is set)
tst r1, #FPEXC_FP2V @ is there an FPINST2 to read?
beq 1f
VFPFMRX r8, FPINST2 @ FPINST2 if needed (and present)
1:
stmia r4, {r1, r5, r6, r8} @ save FPEXC, FPSCR, FPINST, FPINST2
vfp_reload_hw:

#else
@ For SMP, if this thread does not own the hw context, then we
@ need to reload it. No need to save the old state as on SMP,
@ we always save the state when we switch away from a thread.
bne vfp_reload_hw

@ This thread has ownership of the current hardware context.
@ However, it may have been migrated to another CPU, in which
@ case the saved state is newer than the hardware context.
@ Check this by looking at the CPU number which the state was
@ last loaded onto.
ldr ip, [r10, #VFP_CPU]
teq ip, r11
beq vfp_hw_state_valid

vfp_reload_hw:
@ We're loading this threads state into the VFP hardware. Update
@ the CPU number which contains the most up to date VFP context.
str r11, [r10, #VFP_CPU]

VFPFMXR FPEXC, r5 @ enable VFP, disable any pending
@ exceptions, so we can get at the
@ rest of it
#endif

DBGSTR1 "load state %p", r10
str r10, [r9, r11, lsl #2] @ update the vfp_current_hw_state pointer
ENTRY(vfp_load_state)
@ Load the current VFP state
@ r0 - load location
@ returns FPEXC
DBGSTR1 "load VFP state %p", r0
@ Load the saved state back into the VFP
VFPFLDMIA r10, r5 @ reload the working registers while
VFPFLDMIA r0, r1 @ reload the working registers while
@ FPEXC is in a safe state
ldmia r10, {r1, r5, r6, r8} @ load FPEXC, FPSCR, FPINST, FPINST2
tst r1, #FPEXC_EX @ is there additional state to restore?
ldmia r0, {r0-r3} @ load FPEXC, FPSCR, FPINST, FPINST2
tst r0, #FPEXC_EX @ is there additional state to restore?
beq 1f
VFPFMXR FPINST, r6 @ restore FPINST (only if FPEXC.EX is set)
tst r1, #FPEXC_FP2V @ is there an FPINST2 to write?
VFPFMXR FPINST, r2 @ restore FPINST (only if FPEXC.EX is set)
tst r0, #FPEXC_FP2V @ is there an FPINST2 to write?
beq 1f
VFPFMXR FPINST2, r8 @ FPINST2 if needed (and present)
VFPFMXR FPINST2, r3 @ FPINST2 if needed (and present)
1:
VFPFMXR FPSCR, r5 @ restore status

@ The context stored in the VFP hardware is up to date with this thread
vfp_hw_state_valid:
tst r1, #FPEXC_EX
bne process_exception @ might as well handle the pending
@ exception before retrying branch
@ out before setting an FPEXC that
@ stops us reading stuff
VFPFMXR FPEXC, r1 @ Restore FPEXC last
mov sp, r3 @ we think we have handled things
pop {lr}
sub r2, r2, #4 @ Retry current instruction - if Thumb
str r2, [sp, #S_PC] @ mode it's two 16-bit instructions,
@ else it's one 32-bit instruction, so
@ always subtract 4 from the following
@ instruction address.

local_bh_enable_and_ret:
adr r0, .
mov r1, #SOFTIRQ_DISABLE_OFFSET
b __local_bh_enable_ip @ tail call

look_for_VFP_exceptions:
@ Check for synchronous or asynchronous exception
tst r1, #FPEXC_EX | FPEXC_DEX
bne process_exception
@ On some implementations of the VFP subarch 1, setting FPSCR.IXE
@ causes all the CDP instructions to be bounced synchronously without
@ setting the FPEXC.EX bit
VFPFMRX r5, FPSCR
tst r5, #FPSCR_IXE
bne process_exception

tst r5, #FPSCR_LENGTH_MASK
beq skip
orr r1, r1, #FPEXC_DEX
b process_exception
skip:

@ Fall into hand on to next handler - appropriate coproc instr
@ not recognised by VFP

DBGSTR "not VFP"
b local_bh_enable_and_ret

process_exception:
DBGSTR "bounce"
mov sp, r3 @ setup for a return to the user code.
pop {lr}
mov r2, sp @ nothing stacked - regdump is at TOS

@ Now call the C code to package up the bounce to the support code
@ r0 holds the trigger instruction
@ r1 holds the FPEXC value
@ r2 pointer to register dump
b VFP_bounce @ we have handled this - the support
@ code will raise an exception if
@ required. If not, the user code will
@ retry the faulted instruction
ENDPROC(vfp_support_entry)
VFPFMXR FPSCR, r1 @ restore status
ret lr
ENDPROC(vfp_load_state)

ENTRY(vfp_save_state)
@ Save the current VFP state
Expand All @@ -238,10 +66,6 @@ ENTRY(vfp_save_state)
ret lr
ENDPROC(vfp_save_state)

.align
vfp_current_hw_state_address:
.word vfp_current_hw_state

.macro tbl_branch, base, tmp, shift
#ifdef CONFIG_THUMB2_KERNEL
adr \tmp, 1f
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