[PATCH v2 16/43] KVM: Don't redo ktime_get() when calculating halt-polling stop/deadline

Sean Christopherson seanjc at google.com
Fri Oct 8 19:12:09 PDT 2021

Calculate the halt-polling "stop" time using "cur" instead of redoing
ktime_get().  In the happy case where hardware correctly predicts
do_halt_poll, "cur" is only a few cycles old.  And if the branch is
mispredicted, arguably that extra latency should count toward the
halt-polling time.

In all likelihood, the numbers involved are in the noise and either
approach is perfectly ok.

Reviewed-by: David Matlack <dmatlack at google.com>
Signed-off-by: Sean Christopherson <seanjc at google.com>
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index a36ccdc93a72..481e8178b43d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3272,7 +3272,7 @@ void kvm_vcpu_halt(struct kvm_vcpu *vcpu)
 	start = cur = poll_end = ktime_get();
 	if (do_halt_poll) {
-		ktime_t stop = ktime_add_ns(ktime_get(), vcpu->halt_poll_ns);
+		ktime_t stop = ktime_add_ns(cur, vcpu->halt_poll_ns);
 		do {

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