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

[Rik van Riel]

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.ÂÂLet'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:
> 
> ÂÂÂÂ#include <sys/signal.h>
> ÂÂÂÂ#include <time.h>
> ÂÂÂÂ#include <unistd.h>
> ÂÂÂÂ#include <stdio.h>
> 
> ÂÂÂÂunsigned long rdtsc()
> ÂÂÂÂ{
> ÂÂÂÂÂÂÂÂunsigned long result;
> ÂÂÂÂÂÂÂÂasm volatile("rdtsc; shl $32, %%rdx; mov %%eax, %%eax\n"
> ÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂÂ"or %%rdx, %%rax" : "=a" (result) : : "rdx");
> ÂÂÂÂÂÂÂÂreturn result;
> ÂÂÂÂ}
> 
> ÂÂÂÂint main()
> ÂÂÂÂ{
> ÂÂÂÂÂÂÂÂunsigned long tsc1, tsc2;
> ÂÂÂÂÂÂÂÂint pid = getpid();
> ÂÂÂÂÂÂÂÂint i;
> 
> ÂÂÂÂÂÂÂÂtsc1 = rdtsc();
> ÂÂÂÂÂÂÂÂfor (i = 0; i < 100000000; i++)
> ÂÂÂÂÂÂÂÂÂÂÂÂkill(pid, SIGWINCH);
> ÂÂÂÂÂÂÂÂtsc2 = rdtsc();
> 
> ÂÂÂÂÂÂÂÂprintf("%ld\n", tsc2 - tsc1);
> ÂÂÂÂ}
> 
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rik van Riel <riel@xxxxxxxxxx>
> Cc: H. Peter Anvin <hpa@xxxxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>
> 
Reviewed-by: Rik van Riel <riel@xxxxxxxxxx>

-- 
All Rights Reversed.

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