From fb14b4eadf73500d3b2104f031472a268562c047 Mon Sep 17 00:00:00 2001 From: Oleg Nesterov Date: Wed, 11 Mar 2015 18:34:09 +0100 Subject: [PATCH] x86/fpu: Document user_fpu_begin() Currently, user_fpu_begin() has a single caller and it is not clear why do we actually need it and why we should not worry about preemption right after preempt_enable(). Signed-off-by: Oleg Nesterov Signed-off-by: Borislav Petkov Cc: Andy Lutomirski Cc: Dave Hansen Cc: Fenghua Yu Cc: Linus Torvalds Cc: Pekka Riikonen Cc: Quentin Casasnovas Cc: Rik van Riel Cc: Suresh Siddha Link: http://lkml.kernel.org/r/20150311173409.GC5032@redhat.com Signed-off-by: Ingo Molnar --- arch/x86/include/asm/fpu-internal.h | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/arch/x86/include/asm/fpu-internal.h b/arch/x86/include/asm/fpu-internal.h index 810f20fd4e4e..c58c9302152b 100644 --- a/arch/x86/include/asm/fpu-internal.h +++ b/arch/x86/include/asm/fpu-internal.h @@ -508,10 +508,12 @@ static inline int restore_xstate_sig(void __user *buf, int ia32_frame) } /* - * Need to be preemption-safe. + * Needs to be preemption-safe. * * NOTE! user_fpu_begin() must be used only immediately before restoring - * it. This function does not do any save/restore on their own. + * the save state. It does not do any saving/restoring on its own. In + * lazy FPU mode, it is just an optimization to avoid a #NM exception, + * the task can lose the FPU right after preempt_enable(). */ static inline void user_fpu_begin(void) { -- 2.39.2