Commit 11896456 authored by David Matlack's avatar David Matlack Committed by Sean Christopherson
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KVM: Mark a vCPU as preempted/ready iff it's scheduled out while running 



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 54aa83c9 ("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.

Marking a vCPU preempted iff its running 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: default avatarSean Christopherson <seanjc@google.com>
Signed-off-by: default avatarDavid Matlack <dmatlack@google.com>
Link: https://lore.kernel.org/r/20240503181734.1467938-4-dmatlack@google.com


[sean: massage changelog]
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 4b23e0c1

virt/kvm/kvm_main.c

+1 −1
Original line number Diff line number Diff line
@@ -6317,7 +6317,7 @@ static void kvm_sched_out(struct preempt_notifier *pn, 


	WRITE_ONCE(vcpu->scheduled_out, true);



	if (current->on_rq) {

	if (current->on_rq && vcpu->wants_to_run) {

		WRITE_ONCE(vcpu->preempted, true);

		WRITE_ONCE(vcpu->ready, true);

	}