On 10/07/19 18:07, Sean Christopherson wrote:
> 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: f2485b3e0c6c0 ("KVM: x86: use guest_exit_irqoff")
> Reported-by: Wei Yang <w90p710@xxxxxxxxx>
> Signed-off-by: Sean Christopherson <sean.j.christopherson@xxxxxxxxx>
> ---
> 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 @@ 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)
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 2e302e977dac..32561032f7e6 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -8042,7 +8042,18 @@ 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)) {
>
Queued, thanks.
Paolo
