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SubjectRe: [PATCH v3 2/2] x86_64: Don't save flags on context switch
On Wed, Oct 1, 2014 at 11:28 AM, Andy Lutomirski <luto@amacapital.net> 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@amacapital.net>
> ---
> 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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