[PATCH 2/5] KVM: x86: Reset vcpu->arch.st.last_steal when enabling steal time

[Dongli Zhang <dongli.zhang@oracle.com>]

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->arch.st.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->arch.st.last_steal to produce a
large, bogus value.

Fix this by resetting vcpu->arch.st.last_steal to
current->sched_info.run_delay when KVM steal time is enabled.

Signed-off-by: Dongli Zhang <dongli.zhang@oracle.com>
---
 arch/x86/include/asm/kvm_host.h | 1 +
 arch/x86/kvm/x86.c              | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index c470e40a00aa..1f1f29128c5d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -960,6 +960,7 @@ struct kvm_vcpu_arch {
 		u64 msr_val;
 		u64 last_steal;
 		struct gfn_to_hva_cache cache;
+		bool need_reset;
 	} st;
 
 	u64 l1_tsc_offset;
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 0a1b63c63d1a..eec578894ad5 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3829,6 +3829,12 @@ static void record_steal_time(struct kvm_vcpu *vcpu)
 	smp_wmb();
 
 	unsafe_get_user(steal, &st->steal, out);
+
+	if (vcpu->arch.st.need_reset) {
+		vcpu->arch.st.need_reset = false;
+		vcpu->arch.st.last_steal = current->sched_info.run_delay;
+	}
+
 	steal += current->sched_info.run_delay -
 		vcpu->arch.st.last_steal;
 	vcpu->arch.st.last_steal = current->sched_info.run_delay;
@@ -4178,6 +4184,7 @@ int kvm_set_msr_common(struct kvm_vcpu *vcpu, struct msr_data *msr_info)
 		if (!(data & KVM_MSR_ENABLED))
 			break;
 
+		vcpu->arch.st.need_reset = true;
 		kvm_make_request(KVM_REQ_STEAL_UPDATE, vcpu);
 
 		break;
-- 
2.39.3