On Wed, Jan 06, 2021, Vitaly Kuznetsov wrote:
> Nitesh Narayan Lal <nitesh@xxxxxxxxxx> writes:
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 3f7c1fc7a3ce..3e17c9ffcad8 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -9023,18 +9023,7 @@ 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
>
> I kind of liked this part of the comment, the new (old) one in
> svm_handle_exit_irqoff() doesn't actually explain what's going on.
>
> > and any ticks that occur between VM-Exit and now.
>
> Looking back, I don't quite understand why we wanted to account ticks
> between vmexit and exiting guest context as 'guest' in the first place;
> to my understanging 'guest time' is time spent within VMX non-root
> operation, the rest is KVM overhead (system).
With tick-based accounting, if the tick IRQ is received after PF_VCPU is cleared
then that tick will be accounted to the host/system. The motivation for opening
an IRQ window after VM-Exit is to handle the case where the guest is constantly
exiting for a different reason _just_ before the tick arrives, e.g. if the guest
has its tick configured such that the guest and host ticks get synchronized
in a bad way.
This is a non-issue when using CONFIG_VIRT_CPU_ACCOUNTING_GEN=y, at least with a
stable TSC, as the accounting happens during guest_exit_irqoff() itself.
Accounting might be less-than-stellar if TSC is unstable, but I don't think it
would be as binary of a failure as tick-based accounting.
> It seems to match how the accounting is done nowadays after Tglx's
> 87fa7f3e98a1 ("x86/kvm: Move context tracking where it belongs").
>
> > - * 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);
> >
> > if (lapic_in_kernel(vcpu)) {
> > s64 delta = vcpu->arch.apic->lapic_timer.advance_expire_delta;
>
> FWIW,
>
> Reviewed-by: Vitaly Kuznetsov <vkuznets@xxxxxxxxxx>
>
> --
> Vitaly
>
