Re: [PATCH 3.19.y-ckt 130/130] Revert "sched/x86_64: Don't save flags on context switch"

[Andy Lutomirski]

If you haven't pushed this out yet, I would hold off.  I think it may
be unnecessary.

--Andy

On Thu, Aug 27, 2015 at 3:12 PM, Kamal Mostafa <kamal@xxxxxxxxxxxxx> wrote:
> 3.19.8-ckt6 -stable review patch.  If anyone has any objections, please let me know.
>
> ------------------
>
> From: Andy Lutomirski <luto@xxxxxxxxxx>
>
> commit 512255a2ad2c832ca7d4de9f31245f73781922d0 upstream.
>
> This reverts commit:
>
>   2c7577a75837 ("sched/x86_64: Don't save flags on context switch")
>
> It was a nice speedup.  It's also not quite correct: SYSENTER
> enables interrupts too early.
>
> We can re-add this optimization once the SYSENTER code is beaten
> into shape, which should happen in 4.3 or 4.4.
>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Link: http://lkml.kernel.org/r/85f56651f59f76624e80785a8fd3bdfdd089a818.1439838962.git.luto@xxxxxxxxxx
> Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
> Signed-off-by: Kamal Mostafa <kamal@xxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/switch_to.h | 12 ++++--------
>  1 file changed, 4 insertions(+), 8 deletions(-)
>
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index 751bf4b..d7f3b3b 100644
> --- a/arch/x86/include/asm/switch_to.h
> +++ b/arch/x86/include/asm/switch_to.h
> @@ -79,12 +79,12 @@ do {                                                                        \
>  #else /* CONFIG_X86_32 */
>
>  /* frame pointer must be last for get_wchan */
> -#define SAVE_CONTEXT    "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
> -#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
> +#define SAVE_CONTEXT    "pushf ; pushq %%rbp ; movq %%rsi,%%rbp\n\t"
> +#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp ; popf\t"
>
>  #define __EXTRA_CLOBBER  \
>         , "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
> -         "r12", "r13", "r14", "r15", "flags"
> +         "r12", "r13", "r14", "r15"
>
>  #ifdef CONFIG_CC_STACKPROTECTOR
>  #define __switch_canary                                                          \
> @@ -100,11 +100,7 @@ do {                                                                       \
>  #define __switch_canary_iparam
>  #endif /* CC_STACKPROTECTOR */
>
> -/*
> - * There is no need to save or restore flags, because flags are always
> - * clean in kernel mode, with the possible exception of IOPL.  Kernel IOPL
> - * has no effect.
> - */
> +/* Save restore flags to clear handle leaking NT */
>  #define switch_to(prev, next, last) \
>         asm volatile(SAVE_CONTEXT                                         \
>              "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */       \
> --
> 1.9.1
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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