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[v3,3/3] KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running

Message ID 20240503181734.1467938-4-dmatlack@google.com
State Accepted
Headers show
Series KVM: Set vcpu->preempted/ready iff scheduled out while running | expand

Commit Message

David Matlack May 3, 2024, 6:17 p.m. UTC 
Mark a vCPU as preempted/ready if-and-only-if it's scheduled out while
running. i.e. Do not mark a vCPU preempted/ready if it's scheduled out
during a non-KVM_RUN ioctl() or when userspace is doing KVM_RUN with
immediate_exit.

Commit 54aa83c90198 ("KVM: x86: do not set st->preempted when going back
to user space") stopped marking a vCPU as preempted when returning to
userspace, but if userspace then invokes a KVM vCPU ioctl() that gets
preempted, the vCPU will be marked preempted/ready. This is arguably
incorrect behavior since the vCPU was not actually preempted while the
guest was running, it was preempted while doing something on behalf of
userspace.

This commit also avoids KVM dirtying guest memory after userspace has
paused vCPUs, e.g. for Live Migration, which allows userspace to collect
the final dirty bitmap before or in parallel with saving vCPU state
without having to worry about saving vCPU state triggering writes to
guest memory.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: David Matlack <dmatlack@google.com>
---
 virt/kvm/kvm_main.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
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Patch

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 2b29851a90bd..3973e62acc7c 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -6302,7 +6302,7 @@  static void kvm_sched_out(struct preempt_notifier *pn,
 {
 	struct kvm_vcpu *vcpu = preempt_notifier_to_vcpu(pn);
 
-	if (current->on_rq) {
+	if (current->on_rq && vcpu->wants_to_run) {
 		WRITE_ONCE(vcpu->preempted, true);
 		WRITE_ONCE(vcpu->ready, true);
 	}