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 <l...@kernel.org>
> Cc: Peter Zijlstra <pet...@infradead.org>
> Cc: Rik van Riel <r...@redhat.com>
> Cc: H. Peter Anvin <h...@zytor.com>
> Cc: Ingo Molnar <mi...@kernel.org>
> Cc: Thomas Gleixner <t...@linutronix.de>
> Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
>
Reviewed-by: Rik van Riel <r...@redhat.com>-- All Rights Reversed.
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