KVM: x86: Unconditionally enable irqs in guest context (d7a08882) · Commits · 戴 / test

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 <reg>; cli; push <reg>; popf". Fixes: ("KVM: x86: use guest_exit_irqoff") Reported-by:

Wei Yang <w90p710@gmail.com> Signed-off-by:

Sean Christopherson <sean.j.christopherson@intel.com> Signed-off-by:

Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/svm.c

+1 −9

Original line number	Diff line number	Diff line
		@@ -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)

arch/x86/kvm/x86.c

+11 −0

Original line number	Diff line number	Diff line
		@@ -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)) {

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