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

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Message-ID: <1466454115.2756.45.camel@redhat.com>
Date:	Mon, 20 Jun 2016 16:21:55 -0400
From:	Rik van Riel <riel@...hat.com>
To:	Paolo Bonzini <pbonzini@...hat.com>, linux-kernel@...r.kernel.org,
	kvm@...r.kernel.org
Cc:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 1/2] x86/entry: Avoid interrupt flag save and restore

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@...nel.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rik van Riel <riel@...hat.com>
> Cc: H. Peter Anvin <hpa@...or.com>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
> 
Reviewed-by: Rik van Riel <riel@...hat.com>

-- 
All Rights Reversed.


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