Re: [PATCH] KVM: LAPIC: Reset timer_advance_ns if timer mode switch

[Wanpeng Li]

On Thu, 3 Sep 2020 at 05:23, Sean Christopherson
<sean.j.christopherson@xxxxxxxxx> wrote:
>
> On Fri, Aug 28, 2020 at 09:35:08AM +0800, Wanpeng Li wrote:
> > From: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> >
> > per-vCPU timer_advance_ns should be set to 0 if timer mode is not tscdeadline
> > otherwise we waste cpu cycles in the function lapic_timer_int_injected(),
> > especially on AMD platform which doesn't support tscdeadline mode. We can
> > reset timer_advance_ns to the initial value if switch back to tscdealine
> > timer mode.
> >
> > Signed-off-by: Wanpeng Li <wanpengli@xxxxxxxxxxx>
> > ---
> >  arch/x86/kvm/lapic.c | 6 ++++++
> >  1 file changed, 6 insertions(+)
> >
> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> > index 654649b..abc296d 100644
> > --- a/arch/x86/kvm/lapic.c
> > +++ b/arch/x86/kvm/lapic.c
> > @@ -1499,10 +1499,16 @@ static void apic_update_lvtt(struct kvm_lapic *apic)
> >                       kvm_lapic_set_reg(apic, APIC_TMICT, 0);
> >                       apic->lapic_timer.period = 0;
> >                       apic->lapic_timer.tscdeadline = 0;
> > +                     if (timer_mode == APIC_LVT_TIMER_TSCDEADLINE &&
> > +                             lapic_timer_advance_dynamic)
>
> Bad indentation.
>
> > +                             apic->lapic_timer.timer_advance_ns = LAPIC_TIMER_ADVANCE_NS_INIT;
>
> Redoing the tuning seems odd.  Doubt it will matter, but it feels weird to
> have to retune the advancement just because the guest toggled between modes.
>
> Rather than clear timer_advance_ns, can we simply move the check against
> apic->lapic_timer.expired_tscdeadline much earlier?  I think that would
> solve this performance hiccup, and IMO would be a logical change in any
> case.  E.g. with some refactoring to avoid more duplication between VMX and
> SVM

How about something like below:

diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
index 3b32d3b..51ed4f0 100644
--- a/arch/x86/kvm/lapic.c
+++ b/arch/x86/kvm/lapic.c
@@ -1582,9 +1582,6 @@ static void __kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
     struct kvm_lapic *apic = vcpu->arch.apic;
     u64 guest_tsc, tsc_deadline;

-    if (apic->lapic_timer.expired_tscdeadline == 0)
-        return;
-
     tsc_deadline = apic->lapic_timer.expired_tscdeadline;
     apic->lapic_timer.expired_tscdeadline = 0;
     guest_tsc = kvm_read_l1_tsc(vcpu, rdtsc());
@@ -1599,7 +1596,10 @@ static void __kvm_wait_lapic_expire(struct
kvm_vcpu *vcpu)

 void kvm_wait_lapic_expire(struct kvm_vcpu *vcpu)
 {
-    if (lapic_timer_int_injected(vcpu))
+    if (lapic_in_kernel(vcpu) &&
+        vcpu->arch.apic->lapic_timer.expired_tscdeadline &&
+        vcpu->arch.apic->lapic_timer.timer_advance_ns &&
+        lapic_timer_int_injected(vcpu))
         __kvm_wait_lapic_expire(vcpu);
 }
 EXPORT_SYMBOL_GPL(kvm_wait_lapic_expire);
@@ -1635,8 +1635,7 @@ static void apic_timer_expired(struct kvm_lapic
*apic, bool from_timer_fn)
     }

     if (kvm_use_posted_timer_interrupt(apic->vcpu)) {
-        if (apic->lapic_timer.timer_advance_ns)
-            __kvm_wait_lapic_expire(vcpu);
+        kvm_wait_lapic_expire(vcpu);
         kvm_apic_inject_pending_timer_irqs(apic);
         return;
     }
diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index 0194336..19e622a 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3456,9 +3456,7 @@ static __no_kcsan fastpath_t svm_vcpu_run(struct
kvm_vcpu *vcpu)
     clgi();
     kvm_load_guest_xsave_state(vcpu);

-    if (lapic_in_kernel(vcpu) &&
-        vcpu->arch.apic->lapic_timer.timer_advance_ns)
-        kvm_wait_lapic_expire(vcpu);
+    kvm_wait_lapic_expire(vcpu);

     /*
      * If this vCPU has touched SPEC_CTRL, restore the guest's value if
diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index a544351..d6e1656 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6800,9 +6800,7 @@ static fastpath_t vmx_vcpu_run(struct kvm_vcpu *vcpu)
     if (enable_preemption_timer)
         vmx_update_hv_timer(vcpu);

-    if (lapic_in_kernel(vcpu) &&
-        vcpu->arch.apic->lapic_timer.timer_advance_ns)
-        kvm_wait_lapic_expire(vcpu);
+    kvm_wait_lapic_expire(vcpu);

     /*
      * If this vCPU has touched SPEC_CTRL, restore the guest's value if