From d7a08882a0a4b4e176691331ee3f492996579534 Mon Sep 17 00:00:00 2001 From: Sean Christopherson Date: Wed, 10 Jul 2019 09:07:34 -0700 Subject: [PATCH] KVM: x86: Unconditionally enable irqs in guest context On VMX, KVM currently does not re-enable irqs until after it has exited the guest context. As a result, a tick that fires in the window between VM-Exit and guest_exit_irqoff() will be accounted as system time. While said window is relatively small, it's large enough to be problematic in some configurations, e.g. if VM-Exits are consistently occurring a hair earlier than the tick irq. Intentionally toggle irqs back off so that guest_exit_irqoff() can be used in lieu of guest_exit() in order to avoid the save/restore of flags in guest_exit(). On my Haswell system, "nop; cli; sti" is ~6 cycles, versus ~28 cycles for "pushf; pop ; cli; push ; popf". Fixes: f2485b3e0c6c0 ("KVM: x86: use guest_exit_irqoff") Reported-by: Wei Yang Signed-off-by: Sean Christopherson Signed-off-by: Paolo Bonzini --- arch/x86/kvm/svm.c | 10 +--------- arch/x86/kvm/x86.c | 11 +++++++++++ 2 files changed, 12 insertions(+), 9 deletions(-) diff --git a/arch/x86/kvm/svm.c b/arch/x86/kvm/svm.c index 5270711e787f..98b848fcf3e3 100644 --- a/arch/x86/kvm/svm.c +++ b/arch/x86/kvm/svm.c @@ -6184,15 +6184,7 @@ out: static void svm_handle_exit_irqoff(struct kvm_vcpu *vcpu) { - kvm_before_interrupt(vcpu); - local_irq_enable(); - /* - * We must have an instruction with interrupts enabled, so - * the timer interrupt isn't delayed by the interrupt shadow. - */ - asm("nop"); - local_irq_disable(); - kvm_after_interrupt(vcpu); + } static void svm_sched_in(struct kvm_vcpu *vcpu, int cpu) diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c index 81faceba8cec..0347b1e41c40 100644 --- a/arch/x86/kvm/x86.c +++ b/arch/x86/kvm/x86.c @@ -8046,7 +8046,18 @@ static int vcpu_enter_guest(struct kvm_vcpu *vcpu) kvm_x86_ops->handle_exit_irqoff(vcpu); + /* + * Consume any pending interrupts, including the possible source of + * VM-Exit on SVM and any ticks that occur between VM-Exit and now. + * An instruction is required after local_irq_enable() to fully unblock + * interrupts on processors that implement an interrupt shadow, the + * stat.exits increment will do nicely. + */ + kvm_before_interrupt(vcpu); + local_irq_enable(); ++vcpu->stat.exits; + local_irq_disable(); + kvm_after_interrupt(vcpu); guest_exit_irqoff(); if (lapic_in_kernel(vcpu)) {