[PATCH 1/4] KVM: x86: Reset last_steal on vCPU pid change

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 unconditionally to
current->sched_info.run_delay when KVM vCPU PID is changed.

Assisted-by: Codex:GPT-5.5
Signed-off-by: Dongli Zhang <[email protected]>
---
 arch/x86/kvm/Kconfig | 1 +
 arch/x86/kvm/x86.c   | 7 +++++++
 2 files changed, 8 insertions(+)

diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index 801bf9e520db..b7cb2ceda6d9 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -40,6 +40,7 @@ config KVM_X86
 	select HAVE_KVM_MSI
 	select HAVE_KVM_CPU_RELAX_INTERCEPT
 	select HAVE_KVM_NO_POLL
+	select HAVE_KVM_VCPU_RUN_PID_CHANGE
 	select VIRT_XFER_TO_GUEST_WORK
 	select KVM_GENERIC_DIRTYLOG_READ_PROTECT
 	select KVM_VFIO
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 47cb9eba113b..33be45eec32b 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -3717,6 +3717,13 @@ void kvm_service_local_tlb_flush_requests(struct kvm_vcpu *vcpu)
 }
 EXPORT_SYMBOL_FOR_KVM_INTERNAL(kvm_service_local_tlb_flush_requests);
 
+int kvm_arch_vcpu_run_pid_change(struct kvm_vcpu *vcpu)
+{
+	vcpu->arch.st.last_steal = current->sched_info.run_delay;
+
+	return 0;
+}
+
 static void record_steal_time(struct kvm_vcpu *vcpu)
 {
 	struct gfn_to_hva_cache *ghc = &vcpu->arch.st.cache;
-- 
2.43.5