Re: [PATCH v3 2/2] x86_64: Don't save flags on context switch

[Andy Lutomirski]

On Wed, Oct 1, 2014 at 11:28 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> Now that the kernel always runs with clean flags (in particular, NT
> is clear), there is no need to save and restore flags on every
> context switch.

Since I'm liable to forget about this, and it's a nice speedup, I
figured I'd remind you all, too :)

(Really crude benchmarking in KVM: context switches take around 750ns,
and it's maybe 10ns faster with this patch.  Nothing earth-shattering,
but it's still nice.)

--Andy

>
> Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/switch_to.h | 12 ++++++++----
>  1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/x86/include/asm/switch_to.h b/arch/x86/include/asm/switch_to.h
> index d7f3b3b78ac3..751bf4b7bf11 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    "pushf ; pushq %%rbp ; movq %%rsi,%%rbp\n\t"
> -#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp ; popf\t"
> +#define SAVE_CONTEXT    "pushq %%rbp ; movq %%rsi,%%rbp\n\t"
> +#define RESTORE_CONTEXT "movq %%rbp,%%rsi ; popq %%rbp\t"
>
>  #define __EXTRA_CLOBBER  \
>         , "rcx", "rbx", "rdx", "r8", "r9", "r10", "r11", \
> -         "r12", "r13", "r14", "r15"
> +         "r12", "r13", "r14", "r15", "flags"
>
>  #ifdef CONFIG_CC_STACKPROTECTOR
>  #define __switch_canary                                                          \
> @@ -100,7 +100,11 @@ do {                                                                       \
>  #define __switch_canary_iparam
>  #endif /* CC_STACKPROTECTOR */
>
> -/* Save restore flags to clear handle leaking NT */
> +/*
> + * 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.
> + */
>  #define switch_to(prev, next, last) \
>         asm volatile(SAVE_CONTEXT                                         \
>              "movq %%rsp,%P[threadrsp](%[prev])\n\t" /* save RSP */       \
> --
> 1.9.3
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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