On Wed, Jan 09, 2019 at 12:47:30PM +0100, Sebastian Andrzej Siewior wrote: > user_fpu_begin() sets fpu_fpregs_owner_ctx to task's fpu struct. This is > always the case since there is no lazy FPU anymore. > > fpu_fpregs_owner_ctx is used during context switch to decide if it needs > to load the saved registers or if the currently loaded registers are > valid. It could be skipped during > taskA -> kernel thread -> taskA > > because the switch to kernel thread would not alter the CPU's FPU state. > > Since this field is always updated during context switch and never > invalidated, setting it manually (in user context) makes no difference. > A kernel thread with kernel_fpu_begin() block could set > fpu_fpregs_owner_ctx to NULL but a kernel thread does not use > user_fpu_begin(). > This is a leftover from the lazy-FPU time. > > Remove user_fpu_begin(), it does not change fpu_fpregs_owner_ctx's > content. > > Signed-off-by: Sebastian Andrzej Siewior <[email protected]> > --- > arch/x86/include/asm/fpu/internal.h | 17 ----------------- > arch/x86/kernel/fpu/core.c | 4 +--- > arch/x86/kernel/fpu/signal.c | 1 - > 3 files changed, 1 insertion(+), 21 deletions(-)
Reviewed-by: Borislav Petkov <[email protected]> Should we do this microoptimization in addition, to save us the activation when the kernel thread here: taskA -> kernel thread -> taskA doesn't call kernel_fpu_begin() and thus fpu_fpregs_owner_ctx remains the same? It would be a bit more correct as it won't invoke the trace_x86_fpu_regs_activated() TP in case the FPU context is the same. --- diff --git a/arch/x86/include/asm/fpu/internal.h b/arch/x86/include/asm/fpu/internal.h index bfe0bfc7d0d1..ee1ac46a7820 100644 --- a/arch/x86/include/asm/fpu/internal.h +++ b/arch/x86/include/asm/fpu/internal.h @@ -510,7 +510,7 @@ switch_fpu_prepare(struct fpu *old_fpu, int cpu) * Set up the userspace FPU context for the new task, if the task * has used the FPU. */ -static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu) +static inline void switch_fpu_finish(struct fpu *prev_fpu, struct fpu *new_fpu, int cpu) { if (static_cpu_has(X86_FEATURE_FPU)) { if (!fpregs_state_valid(new_fpu, cpu)) { @@ -518,7 +518,8 @@ static inline void switch_fpu_finish(struct fpu *new_fpu, int cpu) copy_kernel_to_fpregs(&new_fpu->state); } - fpregs_activate(new_fpu); + if (prev_fpu != new_fpu) + fpregs_activate(new_fpu); } } diff --git a/arch/x86/kernel/process_32.c b/arch/x86/kernel/process_32.c index 77d9eb43ccac..f8205df2df1d 100644 --- a/arch/x86/kernel/process_32.c +++ b/arch/x86/kernel/process_32.c @@ -290,7 +290,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) this_cpu_write(current_task, next_p); - switch_fpu_finish(next_fpu, cpu); + switch_fpu_finish(prev_fpu, next_fpu, cpu); /* Load the Intel cache allocation PQR MSR. */ resctrl_sched_in(); diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c index ffea7c557963..5f153b963180 100644 --- a/arch/x86/kernel/process_64.c +++ b/arch/x86/kernel/process_64.c @@ -572,7 +572,7 @@ __switch_to(struct task_struct *prev_p, struct task_struct *next_p) this_cpu_write(current_task, next_p); this_cpu_write(cpu_current_top_of_stack, task_top_of_stack(next_p)); - switch_fpu_finish(next_fpu, cpu); + switch_fpu_finish(prev_fpu, next_fpu, cpu); /* Reload sp0. */ update_task_stack(next_p); -- Regards/Gruss, Boris. Good mailing practices for 400: avoid top-posting and trim the reply.
