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x86/entry: Avoid interrupt flag save and restore

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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);
    }

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Reviewed-by: Rik van Riel <riel@redhat.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Brian Gerst <brgerst@gmail.com>
Cc: Denys Vlasenko <dvlasenk@redhat.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: kvm@vger.kernel.org
Link: http://lkml.kernel.org/r/1466434712-31440-2-git-send-email-pbonzini@redhat.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
This commit is contained in:
Paolo Bonzini 2016-06-20 16:58:29 +02:00 • committed by Ingo Molnar
parent 52e31f89cc
commit 2e9d1e150a
2 changed files with 17 additions and 2 deletions
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15
include/linux/context_tracking.h
View file

@ -31,6 +31,19 @@ static inline void user_exit(void)
context_tracking_exit(CONTEXT_USER);
}
/* Called with interrupts disabled. */
static inline void user_enter_irqoff(void)
{
if (context_tracking_is_enabled())
__context_tracking_enter(CONTEXT_USER);
}
static inline void user_exit_irqoff(void)
{
if (context_tracking_is_enabled())
__context_tracking_exit(CONTEXT_USER);
}
static inline enum ctx_state exception_enter(void)
{
enum ctx_state prev_ctx;
@ -69,6 +82,8 @@ static inline enum ctx_state ct_state(void)
#else
static inline void user_enter(void) { }
static inline void user_exit(void) { }
static inline void user_enter_irqoff(void) { }
static inline void user_exit_irqoff(void) { }
static inline enum ctx_state exception_enter(void) { return 0; }
static inline void exception_exit(enum ctx_state prev_ctx) { }
static inline enum ctx_state ct_state(void) { return CONTEXT_DISABLED; }