[PATCH] PRODKERNEL: KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running

[PATCH] PRODKERNEL: KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running

[David Matlack]

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>
---
 include/linux/kvm_host.h | 1 +
 virt/kvm/kvm_main.c      | 5 ++++-
 2 files changed, 5 insertions(+), 1 deletion(-)

diff --git a/include/linux/kvm_host.h b/include/linux/kvm_host.h
index fb6c6109fdca..14b645c12d19 100644
--- a/include/linux/kvm_host.h
+++ b/include/linux/kvm_host.h
@@ -377,6 +377,7 @@ struct kvm_vcpu {
 		bool dy_eligible;
 	} spin_loop;
 #endif
+	bool wants_to_run;
 	bool preempted;
 	bool ready;
 	struct kvm_vcpu_arch arch;
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 486800a7024b..066c6a41a43d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4158,7 +4158,10 @@ static long kvm_vcpu_ioctl(struct file *filp,
 				synchronize_rcu();
 			put_pid(oldpid);
 		}
+		vcpu->wants_to_run = !vcpu->run->immediate_exit;
 		r = kvm_arch_vcpu_ioctl_run(vcpu);
+		vcpu->wants_to_run = false;
+
 		trace_kvm_userspace_exit(vcpu->run->exit_reason, r);
 		break;
 	}
@@ -5968,7 +5971,7 @@ 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);
 	}

base-commit: 45b890f7689eb0aba454fc5831d2d79763781677
-- 
2.43.0.472.g3155946c3a-goog

Re: [PATCH] PRODKERNEL: KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running

[David Matlack]

On Mon, Dec 18, 2023 at 10:58 AM David Matlack <dmatlack@google.com> wrote:
>
> 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.

Sigh. I forgot to drop PRODKERNEL: from the subject line. My apologies.