[PATCH v2 2/4] KVM: Reset last_steal on vCPU pid change

[Dongli Zhang]

KVM does not support vCPU hotplug. When a vCPU is removed, its
corresponding data structures are not freed by KVM. Instead, QEMU destroys
only the userspace state and the vCPU thread, while the KVM vCPU fd remains
open and parked in QEMU.

As a result, vcpu->last_steal is not reset.

If the same vCPU is later re-created by QEMU, last_steal retains its old
value, while current->sched_info.run_delay starts from zero since a new
vCPU thread is created. This causes
current->sched_info.run_delay - vcpu->last_steal to produce a large, bogus
value.

Fix this by resetting vcpu->last_steal to current->sched_info.run_delay
when KVM vCPU PID is changed.

Suggested-by: Sean Christopherson <seanjc at google.com>
Signed-off-by: Dongli Zhang <dongli.zhang at oracle.com>
---
v1->v2:
  - Move from kvm_arch_vcpu_run_pid_change() to its caller.

 virt/kvm/kvm_main.c | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
index 65eb26a0520d..784f0d945b45 100644
--- a/virt/kvm/kvm_main.c
+++ b/virt/kvm/kvm_main.c
@@ -4478,6 +4478,10 @@ static long kvm_vcpu_ioctl(struct file *filp,
 			if (r)
 				break;
 
+#ifdef CONFIG_HAVE_KVM_PV_STEAL_TIME
+			vcpu->last_steal = current->sched_info.run_delay;
+#endif
+
 			newpid = get_task_pid(current, PIDTYPE_PID);
 			write_lock(&vcpu->pid_lock);
 			vcpu->pid = newpid;
-- 
2.43.7