As some architectures (e.g. s390) can't disable preemption while
entering/leaving the guest, they won't receive the yield in all situations.
kvm_enter_guest() has to be called with preemption_disabled and will set
PF_VCPU. After that point e.g. s390 reenables preemption and starts to execute the
guest. The thread might therefore be scheduled out between kvm_enter_guest() and
kvm_exit_guest(), resulting in PF_VCPU being set but not being run.
Please note that preemption has to stay enabled in order to correctly process
page faults on s390.
Current code takes PF_VCPU as a hint that the VCPU thread is running and
therefore needs no yield. yield_to() checks whether the target thread is running,
so let's use the inbuilt functionality to make it independent of PF_VCPU and
preemption.
Signed-off-by: David Hildenbrand <dahi@xxxxxxxxxxxxxxxxxx>
---
virt/kvm/kvm_main.c | 4 ----
1 file changed, 4 deletions(-)
diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 5b45330..184f52e 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -1782,10 +1782,6 @@ int kvm_vcpu_yield_to(struct kvm_vcpu *target)
rcu_read_unlock();
if (!task)
return ret;
- if (task->flags & PF_VCPU) {
- put_task_struct(task);
- return ret;
- }
ret = yield_to(task, 1);
put_task_struct(task);
--
1.8.5.5
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