Re: [PATCH 1/2] x86/entry: Avoid interrupt flag save and restore

Rik van Riel
2016-06-20T20:24:46Z

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On Mon, 2016-06-20 at 16:58 +0200, Paolo Bonzini wrote:
> Thanks to all the work that was done by Andy Lutomirski and others,
> enter_from_user_mode and prepare_exit_to_usermode are now called only
> with
> interrupts disabled.  Let's provide them a version of
> user_enter/user_exit
> that skips saving and restoring the interrupt flag.
>
> On an AMD-based machine I tested this patch on, with force-enabled
> context tracking, the speed-up in system calls was 90 clock cycles or
> 6%,
> measured with the following simple benchmark:
>
>     #include <sys/signal.h>
>     #include <time.h>
>     #include <unistd.h>
>     #include <stdio.h>
>
>     unsigned long rdtsc()
>     {
>         unsigned long result;
>         asm volatile("rdtsc; shl $32, %%rdx; mov %%eax, %%eax\n"
>                      "or %%rdx, %%rax" : "=a" (result) : : "rdx");
>         return result;
>     }
>
>     int main()
>     {
>         unsigned long tsc1, tsc2;
>         int pid = getpid();
>         int i;
>
>         tsc1 = rdtsc();
>         for (i = 0; i < 100000000; i++)
>             kill(pid, SIGWINCH);
>         tsc2 = rdtsc();
>
>         printf("%ld\n", tsc2 - tsc1);
>     }
>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Peter Zijlstra <peterz@infradead.org>
> Cc: Rik van Riel <riel@redhat.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Ingo Molnar <mingo@kernel.org>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
>
Reviewed-by: Rik van Riel <riel@redhat.com>

--
All Rights Reversed.

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