• Paolo Bonzini's avatar
    x86/entry: Avoid interrupt flag save and restore · 2e9d1e15
    Paolo Bonzini authored
    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: default avatarPaolo Bonzini <pbonzini@redhat.com>
    Reviewed-by: default avatarRik van Riel <riel@redhat.com>
    Reviewed-by: default avatarAndy Lutomirski <luto@kernel.org>
    Acked-by: default avatarPaolo 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.comSigned-off-by: default avatarIngo Molnar <mingo@kernel.org>
    2e9d1e15
common.c 13.1 KB