Re: [PATCH v2] KVM: LAPIC: Per vCPU control over kvm_can_post_timer_interrupt

Sean Christopherson
2021-12-16T15:46:03Z

On Thu, Dec 16, 2021, Aili Yao wrote:
> On Tue, 7 Dec 2021 23:23:03 +0000
> Sean Christopherson <seanjc@google.com> wrote:
> > On Tue, Nov 23, 2021 at 10:00 PM Wanpeng Li <kernellwp@gmail.com> wrote:
> > > ---
> > > arch/x86/kvm/lapic.c | 5 ++---
> > > 1 file changed, 2 insertions(+), 3 deletions(-)
> > >
> > > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > > index 759952dd1222..8257566d44c7 100644
> > > --- a/arch/x86/kvm/lapic.c
> > > +++ b/arch/x86/kvm/lapic.c
> > > @@ -113,14 +113,13 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> > >
> > > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> > > {
> > > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > > + return pi_inject_timer && kvm_mwait_in_guest(vcpu->kvm) && kvm_vcpu_apicv_active(vcpu);
> >
> > As Aili's changelog pointed out, MWAIT may not be advertised to the guest.
> >
> > So I think we want this? With a non-functional, opinionated refactoring of
> > kvm_can_use_hv_timer() because I'm terrible at reading !(a || b).
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 40270d7bc597..c77cb386d03d 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -113,14 +113,25 @@ static inline u32 kvm_x2apic_id(struct kvm_lapic *apic)
> >
> > static bool kvm_can_post_timer_interrupt(struct kvm_vcpu *vcpu)
> > {
> > - return pi_inject_timer && kvm_vcpu_apicv_active(vcpu);
> > + return pi_inject_timer && kvm_vcpu_apicv_active(vcpu) &&
> > + (kvm_mwait_in_guest(vcpu) || kvm_hlt_in_guest(vcpu));
> > }
> >
> > bool kvm_can_use_hv_timer(struct kvm_vcpu *vcpu)
> > {
> > - return kvm_x86_ops.set_hv_timer
> > - && !(kvm_mwait_in_guest(vcpu->kvm) ||
> > - kvm_can_post_timer_interrupt(vcpu));
> > + /*
> > + * Don't use the hypervisor timer, a.k.a. VMX Preemption Timer, if the
> > + * guest can execute MWAIT without exiting as the timer will stop
> > + * counting if the core enters C3 or lower. HLT in the guest is ok as
> > + * HLT is effectively C1 and the timer counts in C0, C1, and C2.
> > + *
> > + * Don't use the hypervisor timer if KVM can post a timer interrupt to
> > + * the guest since posted the timer avoids taking an extra a VM-Exit
> > + * when the timer expires.
> > + */
> > + return kvm_x86_ops.set_hv_timer &&
> > + !kvm_mwait_in_guest(vcpu->kvm) &&
> > + !kvm_can_post_timer_interrupt(vcpu));
> > }
> > EXPORT_SYMBOL_GPL(kvm_can_use_hv_timer);
> >
>
> It seems Sean and Wanpeng are busy with some other more important issues;
> So Please let me try to merge Sean, Wanpeng's ideas and suggestions together,also including my opinions
> into one possible approach and get it reviewed, Only if others are OK with this;
>
> I will post a new patch for this later today or tomorrow.

Sorry, I was waiting for someone to say "this works", but never actually said as
much.

Does the above change address your use case? If not, what's missing?