KVM: X86: Reduce the overhead when lapic_timer_advance is disabled

Web:        https://git.kernel.org/torvalds/c/9c48d517ce6da398b8cff0603b75b366759023c4
Commit:     9c48d517ce6da398b8cff0603b75b366759023c4
Parent:     74c55931c71352317ae0f5736ee9e4ca07ba4238
Refname:    refs/heads/master
Author:     Wanpeng Li <[email protected]>
AuthorDate: Fri Dec 1 00:15:10 2017 -0800
Committer:  Paolo Bonzini <[email protected]>
CommitDate: Thu Dec 14 09:26:48 2017 +0100

    KVM: X86: Reduce the overhead when lapic_timer_advance is disabled
    
    When I run ebizzy in a 32 vCPUs guest on a 32 pCPUs Xeon box, I can observe
    ~8000 kvm_wait_lapic_expire CurAvg/s through kvm_stat tool even if the advance
    tscdeadline hrtimer expiration is disabled. Each call to wait_lapic_expire()
    will consume ~70 cycles when a timer fires since apic_timer_expire() will
    set expired_tscdeadline and then wait_lapic_expire() will do some caculation
    before bailing out. So total ~175us per second is lost on this 3.2Ghz machine.
    This patch reduces the overhead by skipping the function wait_lapic_expire()
    when lapic_timer_advance is disabled.
    
    Cc: Paolo Bonzini <[email protected]>
    Cc: Radim Krčmář <[email protected]>
    Signed-off-by: Wanpeng Li <[email protected]>
    Signed-off-by: Paolo Bonzini <[email protected]>
---
 arch/x86/kvm/x86.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 7ea9a57b0684..54d66f238e20 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -7018,7 +7018,8 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu)
 	}
 
 	trace_kvm_entry(vcpu->vcpu_id);
-	wait_lapic_expire(vcpu);
+	if (lapic_timer_advance_ns)
+		wait_lapic_expire(vcpu);
 	guest_enter_irqoff();
 
 	if (unlikely(vcpu->arch.switch_db_regs)) {
--
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