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

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Date:	Mon, 30 May 2016 14:30:13 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	linux-kernel@...r.kernel.org, x86@...nel.org
Cc:	Andy Lutomirski <luto@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>,
	Ingo Molnar <mingo@...nel.org.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: [PATCH 1/2] x86/entry: Avoid interrupt flag save and restore

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.com>
Cc: Thomas Gleixner <tglx@...utronix.de>
Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>
---
 arch/x86/entry/common.c          |  4 ++--
 include/linux/context_tracking.h | 15 +++++++++++++++
 2 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index e79d93d..946bc1a 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -43,7 +43,7 @@ static struct thread_info *pt_regs_to_thread_info(struct pt_regs *regs)
 __visible void enter_from_user_mode(void)
 {
 	CT_WARN_ON(ct_state() != CONTEXT_USER);
-	user_exit();
+	__user_exit();
 }
 #else
 static inline void enter_from_user_mode(void) {}
@@ -274,7 +274,7 @@ __visible inline void prepare_exit_to_usermode(struct pt_regs *regs)
 	ti->status &= ~TS_COMPAT;
 #endif
 
-	user_enter();
+	__user_enter();
 }
 
 #define SYSCALL_EXIT_WORK_FLAGS				\
diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
index d259274..81a1fbc 100644
--- a/include/linux/context_tracking.h
+++ b/include/linux/context_tracking.h
@@ -31,6 +31,19 @@ static inline void user_exit(void)
 		context_tracking_exit(CONTEXT_USER);
 }
 
+/* Called with interrupts disabled.  */
+static inline void __user_enter(void)
+{
+	if (context_tracking_is_enabled())
+		__context_tracking_enter(CONTEXT_USER);
+
+}
+static inline void __user_exit(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(void) { }
+static inline void __user_exit(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; }
-- 
2.4.3