On Wed, Oct 1, 2014 at 11:28 AM, Andy Lutomirski <[email protected]> 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 <[email protected]> > --- > 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 -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to [email protected] More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
