linux-kernel - [PATCH 4/4] KVM: Add a comment explaining the directed yield pending interrupt logic

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Date: Tue,  9 Jan 2024 16:39:38 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Sean Christopherson <seanjc@...gle.com>, Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Like Xu <like.xu.linux@...il.com>
Subject: [PATCH 4/4] KVM: Add a comment explaining the directed yield pending
 interrupt logic

Add a comment to explain why KVM treats vCPUs with pending interrupts as
in-kernel when a vCPU wants to yield to a vCPU that was preempted while
running in kernel mode.

Signed-off-by: Sean Christopherson <seanjc@...gle.com>
---
 virt/kvm/kvm_main.c | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 6326852bfb3d..4a9e7513c585 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4089,6 +4089,13 @@ void kvm_vcpu_on_spin(struct kvm_vcpu *me, bool yield_to_kernel_mode)
 				continue;
 			if (kvm_vcpu_is_blocking(vcpu) && !vcpu_dy_runnable(vcpu))
 				continue;
+
+			/*
+			 * Treat the target vCPU as being in-kernel if it has a
+			 * pending interrupt, as the vCPU trying to yield may
+			 * be spinning waiting on IPI delivery, i.e. the target
+			 * vCPU is in-kernel for the purposes of directed yield.
+			 */
 			if (READ_ONCE(vcpu->preempted) && yield_to_kernel_mode &&
 			    !kvm_arch_dy_has_pending_interrupt(vcpu) &&
 			    !kvm_arch_vcpu_preempted_in_kernel(vcpu))
-- 
2.43.0.472.g3155946c3a-goog