On Tue, May 22, 2018 at 05:05:08PM +0100, Dave Martin wrote:
> Currently the FPSIMD handling code uses the condition task->mm ==
> NULL as a hint that task has no FPSIMD register context.
>
> The ->mm check is only there to filter out tasks that cannot
> possibly have FPSIMD context loaded, for optimisation purposes.
> Also, TIF_FOREIGN_FPSTATE must always be checked anyway before
> saving FPSIMD context back to memory. For these reasons, the ->mm
> checks are not useful, providing that that TIF_FOREIGN_FPSTATE is
> maintained in a consistent way for kernel threads.
>
> This is true by construction however: TIF_FOREIGN_FPSTATE is never
> cleared except when returning to userspace or returning from a
> signal: thus, for a true kernel thread no FPSIMD context is ever
> loaded, TIF_FOREIGN_FPSTATE will remain set and no context will
> ever be saved.
I don't understand this construction proof; from looking at the patch
below it is not obvious to me why fpsimd_thread_switch() can never have
!wrong_task && !wrong_cpu and therefore clear TIF_FOREIGN_FPSTATE for a
kernel thread?
Thanks,
-Christoffer
>
> This patch removes the redundant checks and special-case code.
>
> Signed-off-by: Dave Martin <Dave.Martin@xxxxxxx>
> Cc: Catalin Marinas <catalin.marinas@xxxxxxx>
> Cc: Will Deacon <will.deacon@xxxxxxx>
> Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
>
> ---
>
> Changes since v9:
>
> * New patch. Introduced during debugging, since the ->mm checks
> appear bogus and/or redundant, so are likely to be hiding or
> causing bugs.
> ---
> arch/arm64/include/asm/thread_info.h | 1 +
> arch/arm64/kernel/fpsimd.c | 38 ++++++++++++------------------------
> 2 files changed, 14 insertions(+), 25 deletions(-)
>
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index 740aa03c..a2ac914 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -47,6 +47,7 @@ struct thread_info {
>
> #define INIT_THREAD_INFO(tsk) \
> { \
> + .flags = _TIF_FOREIGN_FPSTATE, \
> .preempt_count = INIT_PREEMPT_COUNT, \
> .addr_limit = KERNEL_DS, \
> }
> diff --git a/arch/arm64/kernel/fpsimd.c b/arch/arm64/kernel/fpsimd.c
> index 3aa100a..1222491 100644
> --- a/arch/arm64/kernel/fpsimd.c
> +++ b/arch/arm64/kernel/fpsimd.c
> @@ -891,31 +891,21 @@ asmlinkage void do_fpsimd_exc(unsigned int esr, struct pt_regs *regs)
>
> void fpsimd_thread_switch(struct task_struct *next)
> {
> + bool wrong_task, wrong_cpu;
> +
> if (!system_supports_fpsimd())
> return;
> - /*
> - * Save the current FPSIMD state to memory, but only if whatever is in
> - * the registers is in fact the most recent userland FPSIMD state of
> - * 'current'.
> - */
> - if (current->mm)
> - fpsimd_save();
>
> - if (next->mm) {
> - /*
> - * If we are switching to a task whose most recent userland
> - * FPSIMD state is already in the registers of *this* cpu,
> - * we can skip loading the state from memory. Otherwise, set
> - * the TIF_FOREIGN_FPSTATE flag so the state will be loaded
> - * upon the next return to userland.
> - */
> - bool wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
> + /* Save unsaved fpsimd state, if any: */
> + fpsimd_save();
> +
> + /* Fix up TIF_FOREIGN_FPSTATE to correctly describe next's state: */
> + wrong_task = __this_cpu_read(fpsimd_last_state.st) !=
> &next->thread.uw.fpsimd_state;
> - bool wrong_cpu = next->thread.fpsimd_cpu != smp_processor_id();
> + 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);
> }
>
> void fpsimd_flush_thread(void)
> @@ -1120,9 +1110,8 @@ void kernel_neon_begin(void)
>
> __this_cpu_write(kernel_neon_busy, true);
>
> - /* Save unsaved task fpsimd state, if any: */
> - if (current->mm)
> - fpsimd_save();
> + /* Save unsaved fpsimd state, if any: */
> + fpsimd_save();
>
> /* Invalidate any task state remaining in the fpsimd regs: */
> fpsimd_flush_cpu_state();
> @@ -1244,8 +1233,7 @@ static int fpsimd_cpu_pm_notifier(struct notifier_block *self,
> {
> switch (cmd) {
> case CPU_PM_ENTER:
> - if (current->mm)
> - fpsimd_save();
> + fpsimd_save();
> fpsimd_flush_cpu_state();
> break;
> case CPU_PM_EXIT:
> --
> 2.1.4
>
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