VCPU TSC synchronization is perfromed in kvm_write_tsc() when the TSC
value being set is within 1 second from the expected, as obtained by
extrapolating of the TSC in already synchronized VCPUs.
This is naturally achieved on all VCPUs at VM start and resume;
however on VCPU hotplug it is not: the newly added VCPU is created
with TSC == 0 while others are well ahead.
To compensate for that, consider host-initiated kvm_write_tsc() with
TSC == 0 a special case requiring synchronization regardless of the
current TSC on other VCPUs.
Signed-off-by: Denis Plotnikov <dplotnikov@xxxxxxxxxxxxx>
Reviewed-by: Roman Kagan <rkagan@xxxxxxxxxxxxx>
---
arch/x86/kvm/x86.c | 29 +++++++++++++++++++----------
1 file changed, 19 insertions(+), 10 deletions(-)
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index c59e38f..cf67091 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -1455,16 +1455,25 @@ void kvm_write_tsc(struct kvm_vcpu *vcpu, struct msr_data *msr)
elapsed = ns - kvm->arch.last_tsc_nsec;
if (vcpu->arch.virtual_tsc_khz) {
- u64 tsc_exp = kvm->arch.last_tsc_write +
- nsec_to_cycles(vcpu, elapsed);
- u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
- /*
- * Special case: TSC write with a small delta (1 second)
- * of virtual cycle time against real time is
- * interpreted as an attempt to synchronize the CPU.
- */
- synchronizing = data < tsc_exp + tsc_hz &&
- data + tsc_hz > tsc_exp;
+ if (data == 0 && msr->host_initiated) {
+ /*
+ * detection of vcpu initialization -- need to sync
+ * with other vCPUs. This particularly helps to keep
+ * kvm_clock stable after CPU hotplug
+ */
+ synchronizing = true;
+ } else {
+ u64 tsc_exp = kvm->arch.last_tsc_write +
+ nsec_to_cycles(vcpu, elapsed);
+ u64 tsc_hz = vcpu->arch.virtual_tsc_khz * 1000LL;
+ /*
+ * Special case: TSC write with a small delta (1 second)
+ * of virtual cycle time against real time is
+ * interpreted as an attempt to synchronize the CPU.
+ */
+ synchronizing = data < tsc_exp + tsc_hz &&
+ data + tsc_hz > tsc_exp;
+ }
}
/*
--
1.8.3.1
