[PATCH v3 20/62] KVM: SVM: Add a comment to explain why avic_vcpu_blocking() ignores IRQ blocking

[Naveen N Rao]

On Mon, Jun 23, 2025 at 09:18:45AM -0700, Sean Christopherson wrote:
> On Mon, Jun 23, 2025, Naveen N Rao wrote:
> > On Wed, Jun 11, 2025 at 03:45:23PM -0700, Sean Christopherson wrote:
> > > Add a comment to explain why KVM clears IsRunning when putting a vCPU,
> > > even though leaving IsRunning=1 would be ok from a functional perspective.
> > > Per Maxim's experiments, a misbehaving VM could spam the AVIC doorbell so
> > > fast as to induce a 50%+ loss in performance.
> > > 
> > > Link: https://lore.kernel.org/all/8d7e0d0391df4efc7cb28557297eb2ec9904f1e5.camel@redhat.com
> > > Cc: Maxim Levitsky <mlevitsk at redhat.com>
> > > Signed-off-by: Sean Christopherson <seanjc at google.com>
> > > ---
> > >  arch/x86/kvm/svm/avic.c | 31 ++++++++++++++++++-------------
> > >  1 file changed, 18 insertions(+), 13 deletions(-)
> > > 
> > > diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
> > > index bf8b59556373..3cf929ac117f 100644
> > > --- a/arch/x86/kvm/svm/avic.c
> > > +++ b/arch/x86/kvm/svm/avic.c
> > > @@ -1121,19 +1121,24 @@ void avic_vcpu_blocking(struct kvm_vcpu *vcpu)
> > >  	if (!kvm_vcpu_apicv_active(vcpu))
> > >  		return;
> > >  
> > > -       /*
> > > -        * Unload the AVIC when the vCPU is about to block, _before_
> > > -        * the vCPU actually blocks.
> > > -        *
> > > -        * Any IRQs that arrive before IsRunning=0 will not cause an
> > > -        * incomplete IPI vmexit on the source, therefore vIRR will also
> > > -        * be checked by kvm_vcpu_check_block() before blocking.  The
> > > -        * memory barrier implicit in set_current_state orders writing
> > > -        * IsRunning=0 before reading the vIRR.  The processor needs a
> > > -        * matching memory barrier on interrupt delivery between writing
> > > -        * IRR and reading IsRunning; the lack of this barrier might be
> > > -        * the cause of errata #1235).
> > > -        */
> > > +	/*
> > > +	 * Unload the AVIC when the vCPU is about to block, _before_ the vCPU
> > > +	 * actually blocks.
> > > +	 *
> > > +	 * Note, any IRQs that arrive before IsRunning=0 will not cause an
> > > +	 * incomplete IPI vmexit on the source; kvm_vcpu_check_block() handles
> > > +	 * this by checking vIRR one last time before blocking.  The memory
> > > +	 * barrier implicit in set_current_state orders writing IsRunning=0
> > > +	 * before reading the vIRR.  The processor needs a matching memory
> > > +	 * barrier on interrupt delivery between writing IRR and reading
> > > +	 * IsRunning; the lack of this barrier might be the cause of errata #1235).
> > > +	 *
> > > +	 * Clear IsRunning=0 even if guest IRQs are disabled, i.e. even if KVM
> > > +	 * doesn't need to detect events for scheduling purposes.  The doorbell
> > 
> > Nit: just IsRunning (you can drop the =0 part).
> 
> Hmm, not really.  It could be:
> 
> 	/* Note, any IRQs that arrive while IsRunning is set will not cause an
> 
> or
> 
> 	/* Note, any IRQs that arrive while IsRunning=1 will not cause an
> 
> but that's just regurgitating the spec.  The slightly more interesting scenario
> that's being described here is what will happen if an IRQ arrives _just_ before
> the below code toggle IsRunning from 1 => 0.

Oh, you're right. I was referring to the last paragraph starting with 
"Clear IsRunning=0 even if", which to me feels like a double negation. 
It reads better if it is "Clear IsRunning even if ..."

> 
> > Trying to understand the significance of IRQs being disabled here. Is 
> > that a path KVM tries to optimize?
> 
> Yep.  KVM doesn't need a notification for the undelivered (virtual) IRQ, because
> it won't be handled by the vCPU until the vCPU enables IRQs, and thus it's not a
> valid wake event for the vCPU.
> 
> So, *if* spurious doorbells didn't affect performance or functionality, then
> ideally KVM would leave IsRunning=1, e.g. so that the IOMMU doesn't need to
> generate GA log events, and so that other vCPUs aren't forced to VM-Exit when
> sending an IPI.  Unfortunately, spurious doorbells are quite intrusive, and so
> KVM "needs" to clear IsRunning.

Ok, got that. kvm_arch_vcpu_blocking() looks like it exists precisely 
for this reason.

Thanks,
Naveen