[PATCH 6/7] KVM: x86: disable preemption around the call to kvm_arch_vcpu_{un|}blocking

Maxim Levitsky
2022-06-06T18:09:24Z

On SVM, if preemption happens right after the call to finish_rcuwait
but before call to kvm_arch_vcpu_unblocking on SVM/AVIC, it itself
will re-enable AVIC, and then we will try to re-enable it again
in kvm_arch_vcpu_unblocking which will lead to a warning
in __avic_vcpu_load.

The same problem can happen if the vCPU is preempted right after the call
to kvm_arch_vcpu_blocking but before the call to prepare_to_rcuwait
and in this case, we will end up with AVIC enabled during sleep -
Ooops.

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
virt/kvm/kvm_main.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5e511fcec80d7..642dba492c65d 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -3337,9 +3337,11 @@ bool kvm_vcpu_block(struct kvm_vcpu *vcpu)

vcpu->stat.generic.blocking = 1;

+ preempt_disable();
kvm_arch_vcpu_blocking(vcpu);
-
prepare_to_rcuwait(wait);
+ preempt_enable();
+
for (;;) {
set_current_state(TASK_INTERRUPTIBLE);

@@ -3349,9 +3351,11 @@ bool kvm_vcpu_block(struct kvm_vcpu *vcpu)
waited = true;
schedule();
}
- finish_rcuwait(wait);

+ preempt_disable();
+ finish_rcuwait(wait);
kvm_arch_vcpu_unblocking(vcpu);
+ preempt_enable();

vcpu->stat.generic.blocking = 0;

--
2.26.3