linux-kernel - [PATCH] x86/paravirt: Avoid setting IF flag, if not necessary

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Date:   Mon,  6 Feb 2017 16:02:41 -0500
From:   Waiman Long <longman@...hat.com>
To:     Jeremy Fitzhardinge <jeremy@...p.org>,
        Chris Wright <chrisw@...s-sol.org>,
        Alok Kataria <akataria@...are.com>,
        Rusty Russell <rusty@...tcorp.com.au>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>
Cc:     x86@...nel.org, virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, Waiman Long <longman@...hat.com>
Subject: [PATCH] x86/paravirt: Avoid setting IF flag, if not necessary

Setting the IF flag can cause an VM exit. So we should avoid touching
the IF flag until absolutely necessary. This patch change the way
the paravirt arch_local_irq_restore() works by checking the previous
flags value and call arch_local_irq_enable() only if the IF flag was
set previously.

On a 32 vCPUs KVM guest running the AIM7 five-sec workload,
the performance increased slightly from 302136.32 jobs/min to
306185.57 (about 1.3%) jobs/min and the %CPU time consumed by
_raw_spin_unlock_irqrestore decreased slightly from 1.68% to 1.53%
with this patch.

Signed-off-by: Waiman Long <longman@...hat.com>
---
 arch/x86/include/asm/paravirt.h | 16 +++++++++++-----
 1 file changed, 11 insertions(+), 5 deletions(-)

diff --git a/arch/x86/include/asm/paravirt.h b/arch/x86/include/asm/paravirt.h
index 1eea6ca..864f57b 100644
--- a/arch/x86/include/asm/paravirt.h
+++ b/arch/x86/include/asm/paravirt.h
@@ -6,6 +6,7 @@
 #ifdef CONFIG_PARAVIRT
 #include <asm/pgtable_types.h>
 #include <asm/asm.h>
+#include <asm/processor-flags.h>
 
 #include <asm/paravirt_types.h>
 
@@ -762,11 +763,6 @@ static inline notrace unsigned long arch_local_save_flags(void)
 	return PVOP_CALLEE0(unsigned long, pv_irq_ops.save_fl);
 }
 
-static inline notrace void arch_local_irq_restore(unsigned long f)
-{
-	PVOP_VCALLEE1(pv_irq_ops.restore_fl, f);
-}
-
 static inline notrace void arch_local_irq_disable(void)
 {
 	PVOP_VCALLEE0(pv_irq_ops.irq_disable);
@@ -786,6 +782,16 @@ static inline notrace unsigned long arch_local_irq_save(void)
 	return f;
 }
 
+/*
+ * In a VM, setting the IF flag can cause an expensive VM exit. So if
+ * interrupt was disabled before arch_local_irq_save(), we don't need to
+ * do anything at all, thus saving an VM exit.
+ */
+static inline notrace void arch_local_irq_restore(unsigned long f)
+{
+	if (f & X86_EFLAGS_IF)
+		arch_local_irq_enable();
+}
 
 /* Make sure as little as possible of this mess escapes. */
 #undef PARAVIRT_CALL
-- 
1.8.3.1