From 586b7ccdb7143b6a9b975d2c6ad52b6ca5c162b9 Mon Sep 17 00:00:00 2001 From: Christian Borntraeger Date: Tue, 28 Jul 2015 15:03:05 +0200 Subject: [PATCH] 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 Reviewed-by: David Hildenbrand Fixes: 785dbef407d8 ("KVM: s390: optimize round trip time in request handling") --- arch/s390/kvm/kvm-s390.c | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c index 2078f92d15ac..f32f843a3631 100644 --- 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.