KVM: x86: disable preemption around the call to kvm_arch_vcpu_{un|}blocking

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>
Message-Id: <20220606180829.102503-7-mlevitsk@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

Author: Maxim Levitsky

virt/kvm/kvm_main.c

@@ -3328,9 +3328,11 @@ 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);
 
@@ -3340,9 +3342,11 @@ 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;