KVM: SVM: Add a comment to explain why avic_vcpu_blocking() ignores IRQ blocking

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@redhat.com>
Acked-by: Naveen N Rao (AMD) <naveen@kernel.org>
Link: https://lore.kernel.org/r/20250611224604.313496-22-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

Author: sean-jc

arch/x86/kvm/svm/avic.c

@@ -1122,19 +1122,24 @@ 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
+	 * used to signal running vCPUs cannot be blocked, i.e. will perturb the
+	 * CPU and cause noisy neighbor problems if the VM is sending interrupts
+	 * to the vCPU while it's scheduled out.
+	 */
 	avic_vcpu_put(vcpu);
 }