[PATCH v3 2/5] arm64: fpsimd: Preserve/restore kernel mode NEON at context switch
Mark Rutland
mark.rutland at arm.com
Mon Nov 27 05:09:51 PST 2023
Hi Ard,
On Mon, Nov 27, 2023 at 01:23:02PM +0100, Ard Biesheuvel wrote:
> From: Ard Biesheuvel <ardb at kernel.org>
>
> Currently, the FPSIMD register file is not preserved and restored along
> with the general registers on exception entry/exit or context switch.
> For this reason, we disable preemption when enabling FPSIMD for kernel
> mode use in task context, and suspend the processing of softirqs so that
> there are no concurrent uses in the kernel. (Kernel mode FPSIMD may not
> be used at all in other contexts).
>
> Disabling preemption while doing CPU intensive work on inputs of
> potentially unbounded size is bad for real-time performance, which is
> why we try and ensure that SIMD crypto code does not operate on more
> than ~4k at a time, which is an arbitrary limit and requires assembler
> code to implement efficiently.
>
> We can avoid the need for disabling preemption if we can ensure that any
> in-kernel users of the NEON will not lose the FPSIMD register state
> across a context switch. And given that disabling softirqs implicitly
> disables preemption as well, we will also have to ensure that a softirq
> that runs code using FPSIMD can safely interrupt an in-kernel user.
>
> So introduce a thread_info flag TIF_USING_KMODE_FPSIMD, and modify the
> context switch hook for FPSIMD to preserve and restore the kernel mode
> FPSIMD to/from struct thread_struct when it is set. This avoids any
> scheduling blackouts due to prolonged use of FPSIMD in kernel mode,
> without the need for manual yielding.
>
> In order to support softirq processing while FPSIMD is being used in
> kernel task context, use the same flag to decide whether the kernel mode
> FPSIMD state needs to be preserved and restored before allowing FPSIMD
> to be used in softirq context.
>
> Signed-off-by: Ard Biesheuvel <ardb at kernel.org>
> Reviewed-by: Mark Brown <broonie at kernel.org>
I have a couple of naming/structural comments below, but this looks
functionally good to me. I appreciate those are arguable bikeshedding, so
either way:
Reviewed-by: Mark Rutland <mark.rutland at arm.com>
> ---
> arch/arm64/include/asm/processor.h | 2 +
> arch/arm64/include/asm/thread_info.h | 1 +
> arch/arm64/kernel/fpsimd.c | 92 ++++++++++++++++----
> 3 files changed, 77 insertions(+), 18 deletions(-)
>
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index e5bc54522e71..dcb51c0571af 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -167,6 +167,8 @@ struct thread_struct {
> unsigned long fault_address; /* fault info */
> unsigned long fault_code; /* ESR_EL1 value */
> struct debug_info debug; /* debugging */
> +
> + struct user_fpsimd_state kmode_fpsimd_state;
> #ifdef CONFIG_ARM64_PTR_AUTH
> struct ptrauth_keys_user keys_user;
> #ifdef CONFIG_ARM64_PTR_AUTH_KERNEL
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index 553d1bc559c6..6b254cf90e8b 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -80,6 +80,7 @@ void arch_setup_new_exec(void);
> #define TIF_TAGGED_ADDR 26 /* Allow tagged user addresses */
> #define TIF_SME 27 /* SME in use */
> #define TIF_SME_VL_INHERIT 28 /* Inherit SME vl_onexec across exec */
> +#define TIF_USING_KMODE_FPSIMD 29 /* Task is in a kernel mode FPSIMD section */
Sorry for the bikeshedding, but "KMODE" isn't a term we use elsewhere, and I
think it'd be nicer/clearer if this had "KERNEL" spelled out in full,
especially as it's only 1 additional character.
Could this be TIF_FPSIMD_KERNEL, or maybe TIF_KERNEL_FPSTATE to align with
TIF_FORIEGN_FPSTATE?
>
> #define _TIF_SIGPENDING (1 << TIF_SIGPENDING)
> #define _TIF_NEED_RESCHED (1 << TIF_NEED_RESCHED)
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index ccc4a78a70e4..198918805bf6 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -357,6 +357,7 @@ static void task_fpsimd_load(void)
>
> WARN_ON(!system_supports_fpsimd());
> WARN_ON(preemptible());
> + WARN_ON(test_thread_flag(TIF_USING_KMODE_FPSIMD));
>
> if (system_supports_sve() || system_supports_sme()) {
> switch (current->thread.fp_type) {
> @@ -379,7 +380,7 @@ static void task_fpsimd_load(void)
> default:
> /*
> * This indicates either a bug in
> - * fpsimd_save() or memory corruption, we
> + * fpsimd_save_user_state() or memory corruption, we
> * should always record an explicit format
> * when we save. We always at least have the
> * memory allocated for FPSMID registers so
> @@ -430,7 +431,7 @@ static void task_fpsimd_load(void)
> * than via current, if we are saving KVM state then it will have
> * ensured that the type of registers to save is set in last->to_save.
> */
> -static void fpsimd_save(void)
> +static void fpsimd_save_user_state(void)
> {
> struct cpu_fp_state const *last =
> this_cpu_ptr(&fpsimd_last_state);
> @@ -861,7 +862,7 @@ int vec_set_vector_length(struct task_struct *task, enum vec_type type,
> if (task == current) {
> get_cpu_fpsimd_context();
>
> - fpsimd_save();
> + fpsimd_save_user_state();
> }
>
> fpsimd_flush_task_state(task);
> @@ -1473,6 +1474,16 @@ void do_fpsimd_exc(unsigned long esr, struct pt_regs *regs)
> current);
> }
>
> +static void fpsimd_load_kernel_state(struct task_struct *task)
> +{
> + fpsimd_load_state(&task->thread.kmode_fpsimd_state);
> +}
> +
> +static void fpsimd_save_kernel_state(struct task_struct *task)
> +{
> + fpsimd_save_state(&task->thread.kmode_fpsimd_state);
> +}
> +
> void fpsimd_thread_switch(struct task_struct *next)
> {
> bool wrong_task, wrong_cpu;
> @@ -1483,19 +1494,28 @@ void fpsimd_thread_switch(struct task_struct *next)
> WARN_ON_ONCE(!irqs_disabled());
>
> /* Save unsaved fpsimd state, if any: */
> - fpsimd_save();
> + if (!test_thread_flag(TIF_USING_KMODE_FPSIMD))
> + fpsimd_save_user_state();
> + else
> + fpsimd_save_kernel_state(current);
Minor nit: I find this condition inversion slightly hard to read since in prose
it'd be "if there's not kernel state, save the user state; else save the kernel
state", whereas:
if (test_thread_flag(TIF_USING_KMODE_FPSIMD))
fpsimd_save_kernel_state(current);
else
fpsimd_save_user_state();
... is more clearly "if there's kernel state, save it; else save the user
state", and I think that'd be preferable.
>
> - /*
> - * Fix up TIF_FOREIGN_FPSTATE to correctly describe next's
> - * state. For kernel threads, FPSIMD registers are never loaded
> - * and wrong_task and wrong_cpu will always be true.
> - */
> - wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
> - &next->thread.uw.fpsimd_state;
> - wrong_cpu = next->thread.fpsimd_cpu != smp_processor_id();
> + if (test_tsk_thread_flag(next, TIF_USING_KMODE_FPSIMD)) {
> + fpsimd_load_kernel_state(next);
> + set_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE);
> + } else {
> + /*
> + * Fix up TIF_FOREIGN_FPSTATE to correctly describe next's
> + * state. For kernel threads, FPSIMD registers are never
> + * loaded with user mode FPSIMD state and so wrong_task and
> + * wrong_cpu will always be true.
> + */
> + wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
> + &next->thread.uw.fpsimd_state;
> + wrong_cpu = next->thread.fpsimd_cpu != smp_processor_id();
>
> - update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE,
> - wrong_task || wrong_cpu);
> + update_tsk_thread_flag(next, TIF_FOREIGN_FPSTATE,
> + wrong_task || wrong_cpu);
> + }
> }
>
> static void fpsimd_flush_thread_vl(enum vec_type type)
> @@ -1585,7 +1605,7 @@ void fpsimd_preserve_current_state(void)
> return;
>
> get_cpu_fpsimd_context();
> - fpsimd_save();
> + fpsimd_save_user_state();
> put_cpu_fpsimd_context();
> }
>
> @@ -1801,7 +1821,7 @@ void fpsimd_save_and_flush_cpu_state(void)
> return;
> WARN_ON(preemptible());
> get_cpu_fpsimd_context();
> - fpsimd_save();
> + fpsimd_save_user_state();
> fpsimd_flush_cpu_state();
> put_cpu_fpsimd_context();
> }
> @@ -1835,10 +1855,37 @@ void kernel_neon_begin(void)
> get_cpu_fpsimd_context();
>
> /* Save unsaved fpsimd state, if any: */
> - fpsimd_save();
> + if (!test_thread_flag(TIF_USING_KMODE_FPSIMD)) {
> + fpsimd_save_user_state();
> +
> + /*
> + * Set the thread flag so that the kernel mode FPSIMD state
> + * will be context switched along with the rest of the task
> + * state.
> + *
> + * On non-PREEMPT_RT, softirqs may interrupt task level kernel
> + * mode FPSIMD, but the task will not be preemptible so setting
> + * TIF_USING_KMODE_FPSIMD for those would be both wrong (as it
> + * would mark the task context FPSIMD state as requiring a
> + * context switch) and unnecessary.
> + *
> + * On PREEMPT_RT, softirqs are serviced from a separate thread,
> + * which is scheduled as usual, and this guarantees that these
> + * softirqs are not interrupting use of the FPSIMD in kernel
> + * mode in task context. So in this case, setting the flag here
> + * is always appropriate.
> + */
> + if (IS_ENABLED(CONFIG_PREEMPT_RT) || !in_serving_softirq())
> + set_thread_flag(TIF_USING_KMODE_FPSIMD);
> + } else {
> + BUG_ON(IS_ENABLED(CONFIG_PREEMPT_RT) || !in_serving_softirq());
> + fpsimd_save_kernel_state(current);
> + }
Same comment as above for condition inversion here.
Mark.
>
> /* Invalidate any task state remaining in the fpsimd regs: */
> fpsimd_flush_cpu_state();
> +
> + put_cpu_fpsimd_context();
> }
> EXPORT_SYMBOL_GPL(kernel_neon_begin);
>
> @@ -1856,7 +1903,16 @@ void kernel_neon_end(void)
> if (!system_supports_fpsimd())
> return;
>
> - put_cpu_fpsimd_context();
> + /*
> + * If we are returning from a nested use of kernel mode FPSIMD, restore
> + * the task context kernel mode FPSIMD state. This can only happen when
> + * running in softirq context on non-PREEMPT_RT.
> + */
> + if (!IS_ENABLED(CONFIG_PREEMPT_RT) && in_serving_softirq() &&
> + test_thread_flag(TIF_USING_KMODE_FPSIMD))
> + fpsimd_load_kernel_state(current);
> + else
> + clear_thread_flag(TIF_USING_KMODE_FPSIMD);
> }
> EXPORT_SYMBOL_GPL(kernel_neon_end);
>
> --
> 2.43.0.rc1.413.gea7ed67945-goog
>
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