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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: f2485b3e0c ("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>
This commit is contained in:
Sean Christopherson 2019-07-10 09:07:34 -07:00 committed by Paolo Bonzini
parent 66bb8a065f
commit d7a08882a0
2 changed files with 12 additions and 9 deletions
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10
arch/x86/kvm/svm.c
View File

@ -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)

11
arch/x86/kvm/x86.c
View File

@ -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)) {