KVM: s390: Fix hang VCPU hang/loop regression

commit 785dbef407d8 ("KVM: s390: optimize round trip time in request
handling") introduced a regression. This regression was seen with
CPU hotplug in the guest and switching between 1 or 2 CPUs. This will
set/reset the IBS control via synced request.

Whenever we make a synced request, we first set the vcpu->requests
bit and then block the vcpu. The handler, on the other hand, unblocks
itself, processes vcpu->requests (by clearing them) and unblocks itself
once again.

Now, if the requester sleeps between setting of vcpu->requests and
blocking, the handler will clear the vcpu->requests bit and try to
unblock itself (although no bit is set). When the requester wakes up,
it blocks the VCPU and we have a blocked VCPU without requests.

Solution is to always unset the block bit.

Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com>
Reviewed-by: David Hildenbrand <dahi@linux.vnet.ibm.com>
Fixes: 785dbef407d8 ("KVM: s390: optimize round trip time in request handling")

diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
index 2078f92d15ac90adcfec617b46df750e073b76d0..f32f843a3631359e49b88169ab8a1eed2b76b946 100644 (file)
--- a/arch/s390/kvm/kvm-s390.c
+++ b/arch/s390/kvm/kvm-s390.c
@@ -1742,10 +1742,10 @@ static bool ibs_enabled(struct kvm_vcpu *vcpu)
 
 static int kvm_s390_handle_requests(struct kvm_vcpu *vcpu)
 {
-       if (!vcpu->requests)
-               return 0;
 retry:
        kvm_s390_vcpu_request_handled(vcpu);
+       if (!vcpu->requests)
+               return 0;
        /*
         * We use MMU_RELOAD just to re-arm the ipte notifier for the
         * guest prefix page. gmap_ipte_notify will wait on the ptl lock.