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

On Mon, Jun 23, 2025 at 09:18:45AM -0700, Sean Christopherson wrote:

On Mon, Jun 23, 2025, Naveen N Rao wrote:

quoted

On Wed, Jun 11, 2025 at 03:45:23PM -0700, Sean Christopherson wrote:

quoted

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 (local)
Cc: Maxim Levitsky <redacted>
Signed-off-by: Sean Christopherson <seanjc@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 ..."

quoted

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