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@xxxxxxxxxx>
---
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
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