| Message ID | 20221207103936.2198407-2-ardb@kernel.org (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | ARM: allow kernel mode NEON in softirq context | expand |
On Wed, Dec 7, 2022 at 11:39 AM Ard Biesheuvel <ardb@kernel.org> wrote: > In a subsequent patch, we will relax the kernel mode NEON policy, and > permit kernel mode NEON to be used not only from task context, as is > permitted today, but also from softirq context. > > Given that softirqs may trigger over the back of any IRQ unless they are > explicitly disabled, we need to address the resulting races in the VFP > state handling, by disabling softirq processing in two distinct but > related cases: > - kernel mode NEON will leave the FPU disabled after it completes, so > any kernel code sequence that enables the FPU and subsequently accesses > its registers needs to disable softirqs until it completes; > - kernel_neon_begin() will preserve the userland VFP state in memory, > and if it interrupts the ordinary VFP state preserve sequence, the > latter will resume execution with the VFP registers corrupted, and > happily save them to memory. > > Given that disabling softirqs also disables preemption, we can replace > the existing preempt_disable/enable occurrences in the VFP state > handling asm code with new macros that dis/enable softirqs instead. > In the VFP state handling C code, add local_bh_disable/enable() calls > in those places where the VFP state is preserved. > > One thing to keep in mind is that, once we allow NEON use in softirq > context, the result of any such interruption is that the FPEXC_EN bit in > the FPEXC register will be cleared, and vfp_current_hw_state[cpu] will > be NULL. This means that any sequence that [conditionally] clears > FPEXC_EN and/or sets vfp_current_hw_state[cpu] to NULL does not need to > run with softirqs disabled, as the result will be the same. Furthermore, > the handling of THREAD_NOTIFY_SWITCH is guaranteed to run with IRQs > disabled, and so it does not need protection from softirq interruptions > either. > > Signed-off-by: Ard Biesheuvel <ardb@kernel.org> Tricky patch, I had to read it a few times and visualize the concepts, but I am sufficiently convinced that it does the right thing. Reviewed-by: Linus Walleij <linus.walleij@linaro.org> Yours, Linus Walleij
diff --git a/arch/arm/include/asm/assembler.h b/arch/arm/include/asm/assembler.h index 90fbe4a3f9c8472f..df999b75c0e25b01 100644 --- a/arch/arm/include/asm/assembler.h +++ b/arch/arm/include/asm/assembler.h @@ -236,21 +236,26 @@ THUMB( fpreg .req r7 ) sub \tmp, \tmp, #1 @ decrement it str \tmp, [\ti, #TI_PREEMPT] .endm - - .macro dec_preempt_count_ti, ti, tmp - get_thread_info \ti - dec_preempt_count \ti, \tmp - .endm #else .macro inc_preempt_count, ti, tmp .endm .macro dec_preempt_count, ti, tmp .endm +#endif + + .macro local_bh_disable, ti, tmp + ldr \tmp, [\ti, #TI_PREEMPT] + add \tmp, \tmp, #SOFTIRQ_DISABLE_OFFSET + str \tmp, [\ti, #TI_PREEMPT] + .endm - .macro dec_preempt_count_ti, ti, tmp + .macro local_bh_enable_ti, ti, tmp + get_thread_info \ti + ldr \tmp, [\ti, #TI_PREEMPT] + sub \tmp, \tmp, #SOFTIRQ_DISABLE_OFFSET + str \tmp, [\ti, #TI_PREEMPT] .endm -#endif #define USERL(l, x...) \ 9999: x; \ diff --git a/arch/arm/kernel/asm-offsets.c b/arch/arm/kernel/asm-offsets.c index 2c8d76fd7c66298a..38121c59cbc26cdd 100644 --- a/arch/arm/kernel/asm-offsets.c +++ b/arch/arm/kernel/asm-offsets.c @@ -56,6 +56,7 @@ int main(void) DEFINE(VFP_CPU, offsetof(union vfp_state, hard.cpu)); #endif #endif + DEFINE(SOFTIRQ_DISABLE_OFFSET,SOFTIRQ_DISABLE_OFFSET); #ifdef CONFIG_ARM_THUMBEE DEFINE(TI_THUMBEE_STATE, offsetof(struct thread_info, thumbee_state)); #endif diff --git a/arch/arm/vfp/entry.S b/arch/arm/vfp/entry.S index 27b0a1f27fbdf392..9a89264cdcc0b46e 100644 --- a/arch/arm/vfp/entry.S +++ b/arch/arm/vfp/entry.S @@ -22,7 +22,7 @@ @ IRQs enabled. @ ENTRY(do_vfp) - inc_preempt_count r10, r4 + local_bh_disable r10, r4 ldr r4, .LCvfp ldr r11, [r10, #TI_CPU] @ CPU number add r10, r10, #TI_VFPSTATE @ r10 = workspace @@ -30,7 +30,7 @@ ENTRY(do_vfp) ENDPROC(do_vfp) ENTRY(vfp_null_entry) - dec_preempt_count_ti r10, r4 + local_bh_enable_ti r10, r4 ret lr ENDPROC(vfp_null_entry) diff --git a/arch/arm/vfp/vfphw.S b/arch/arm/vfp/vfphw.S index 6f7926c9c1790f66..26c4f61ecfa39638 100644 --- a/arch/arm/vfp/vfphw.S +++ b/arch/arm/vfp/vfphw.S @@ -175,7 +175,7 @@ vfp_hw_state_valid: @ else it's one 32-bit instruction, so @ always subtract 4 from the following @ instruction address. - dec_preempt_count_ti r10, r4 + local_bh_enable_ti r10, r4 ret r9 @ we think we have handled things @@ -200,7 +200,7 @@ skip: @ not recognised by VFP DBGSTR "not VFP" - dec_preempt_count_ti r10, r4 + local_bh_enable_ti r10, r4 ret lr process_exception: diff --git a/arch/arm/vfp/vfpmodule.c b/arch/arm/vfp/vfpmodule.c index 2cb355c1b5b71694..8f5bc672b4aac04a 100644 --- a/arch/arm/vfp/vfpmodule.c +++ b/arch/arm/vfp/vfpmodule.c @@ -416,7 +416,7 @@ void VFP_bounce(u32 trigger, u32 fpexc, struct pt_regs *regs) if (exceptions) vfp_raise_exceptions(exceptions, trigger, orig_fpscr, regs); exit: - preempt_enable(); + local_bh_enable(); } static void vfp_enable(void *unused) @@ -517,6 +517,8 @@ void vfp_sync_hwstate(struct thread_info *thread) { unsigned int cpu = get_cpu(); + local_bh_disable(); + if (vfp_state_in_hw(cpu, thread)) { u32 fpexc = fmrx(FPEXC); @@ -528,6 +530,7 @@ void vfp_sync_hwstate(struct thread_info *thread) fmxr(FPEXC, fpexc); } + local_bh_enable(); put_cpu(); } @@ -717,6 +720,8 @@ void kernel_neon_begin(void) unsigned int cpu; u32 fpexc; + local_bh_disable(); + /* * Kernel mode NEON is only allowed outside of interrupt context * with preemption disabled. This will make sure that the kernel @@ -739,6 +744,7 @@ void kernel_neon_begin(void) vfp_save_state(vfp_current_hw_state[cpu], fpexc); #endif vfp_current_hw_state[cpu] = NULL; + local_bh_enable(); } EXPORT_SYMBOL(kernel_neon_begin);
In a subsequent patch, we will relax the kernel mode NEON policy, and permit kernel mode NEON to be used not only from task context, as is permitted today, but also from softirq context. Given that softirqs may trigger over the back of any IRQ unless they are explicitly disabled, we need to address the resulting races in the VFP state handling, by disabling softirq processing in two distinct but related cases: - kernel mode NEON will leave the FPU disabled after it completes, so any kernel code sequence that enables the FPU and subsequently accesses its registers needs to disable softirqs until it completes; - kernel_neon_begin() will preserve the userland VFP state in memory, and if it interrupts the ordinary VFP state preserve sequence, the latter will resume execution with the VFP registers corrupted, and happily save them to memory. Given that disabling softirqs also disables preemption, we can replace the existing preempt_disable/enable occurrences in the VFP state handling asm code with new macros that dis/enable softirqs instead. In the VFP state handling C code, add local_bh_disable/enable() calls in those places where the VFP state is preserved. One thing to keep in mind is that, once we allow NEON use in softirq context, the result of any such interruption is that the FPEXC_EN bit in the FPEXC register will be cleared, and vfp_current_hw_state[cpu] will be NULL. This means that any sequence that [conditionally] clears FPEXC_EN and/or sets vfp_current_hw_state[cpu] to NULL does not need to run with softirqs disabled, as the result will be the same. Furthermore, the handling of THREAD_NOTIFY_SWITCH is guaranteed to run with IRQs disabled, and so it does not need protection from softirq interruptions either. Signed-off-by: Ard Biesheuvel <ardb@kernel.org> --- arch/arm/include/asm/assembler.h | 19 ++++++++++++------- arch/arm/kernel/asm-offsets.c | 1 + arch/arm/vfp/entry.S | 4 ++-- arch/arm/vfp/vfphw.S | 4 ++-- arch/arm/vfp/vfpmodule.c | 8 +++++++- 5 files changed, 24 insertions(+), 12 deletions(-)