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.

Link: https://lore.kernel.org/r/20240110003938.490206-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 9b92858c8b72acfac046b4fe86193042be32c971..c0c2fcdabe1b3b0a369e3e1f9e675c32c9bdf592 100644 (file)
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4090,6 +4090,13 @@ 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))