KVM: x86: do not set st->preempted when going back to user space

Similar to the Xen path, only change the vCPU's reported state if the vCPU
was actually preempted.  The reason for KVM's behavior is that for example
optimistic spinning might not be a good idea if the guest is doing repeated
exits to userspace; however, it is confusing and unlikely to make a difference,
because well-tuned guests will hardly ever exit KVM_RUN in the first place.

Suggested-by: Sean Christopherson <seanjc@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index b81ef4f..a8bb635 100644 (file)
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -4654,19 +4654,21 @@ void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
 {
        int idx;
 
-       if (vcpu->preempted && !vcpu->arch.guest_state_protected)
-               vcpu->arch.preempted_in_kernel = !static_call(kvm_x86_get_cpl)(vcpu);
+       if (vcpu->preempted) {
+               if (!vcpu->arch.guest_state_protected)
+                       vcpu->arch.preempted_in_kernel = !static_call(kvm_x86_get_cpl)(vcpu);
 
-       /*
-        * Take the srcu lock as memslots will be accessed to check the gfn
-        * cache generation against the memslots generation.
-        */
-       idx = srcu_read_lock(&vcpu->kvm->srcu);
-       if (kvm_xen_msr_enabled(vcpu->kvm))
-               kvm_xen_runstate_set_preempted(vcpu);
-       else
-               kvm_steal_time_set_preempted(vcpu);
-       srcu_read_unlock(&vcpu->kvm->srcu, idx);
+               /*
+                * Take the srcu lock as memslots will be accessed to check the gfn
+                * cache generation against the memslots generation.
+                */
+               idx = srcu_read_lock(&vcpu->kvm->srcu);
+               if (kvm_xen_msr_enabled(vcpu->kvm))
+                       kvm_xen_runstate_set_preempted(vcpu);
+               else
+                       kvm_steal_time_set_preempted(vcpu);
+               srcu_read_unlock(&vcpu->kvm->srcu, idx);
+       }
 
        static_call(kvm_x86_vcpu_put)(vcpu);
        vcpu->arch.last_host_tsc = rdtsc();

diff --git a/arch/x86/kvm/xen.h b/arch/x86/kvm/xen.h
index ee5c4ae..532a535 100644 (file)
--- a/arch/x86/kvm/xen.h
+++ b/arch/x86/kvm/xen.h
@@ -159,8 +159,10 @@ static inline void kvm_xen_runstate_set_preempted(struct kvm_vcpu *vcpu)
         * behalf of the vCPU. Only if the VMM does actually block
         * does it need to enter RUNSTATE_blocked.
         */
-       if (vcpu->preempted)
-               kvm_xen_update_runstate_guest(vcpu, RUNSTATE_runnable);
+       if (WARN_ON_ONCE(!vcpu->preempted))
+               return;
+
+       kvm_xen_update_runstate_guest(vcpu, RUNSTATE_runnable);
 }
 
 /* 32-bit compatibility definitions, also used natively in 32-bit build */