Commit d7a08882 authored by Sean Christopherson's avatar Sean Christopherson Committed by Paolo Bonzini
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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 <reg>; cli; push <reg>; popf".

Fixes: f2485b3e ("KVM: x86: use guest_exit_irqoff")
Reported-by: default avatarWei Yang <w90p710@gmail.com>
Signed-off-by: default avatarSean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: default avatarPaolo Bonzini <pbonzini@redhat.com>
parent 66bb8a06

arch/x86/kvm/svm.c

+1 −9
Original line number Diff line number Diff line
@@ -6184,15 +6184,7 @@ static int svm_check_intercept(struct kvm_vcpu *vcpu, 


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)) {