Bug 217424

Hi

We are using lightweight VM with snapshot feature, the VM will be saved with
100ms+, and we found restore such VM will not get correct TSC, which will make
the VM world stop about 100ms+ after restore (the stop time is same as time
when VM saved).

After Investigation, we found the issue caused by TSC synchronization in
setting MSR_IA32_TSC. In VM save, VMM (cloud-hypervisor) will record TSC of each
VCPU, then restore the TSC of VCPU in VM restore (about 100ms+ in guest time).
But in KVM, setting a TSC within 1 second is identified as TSC synchronization,
and the TSC offset will not be updated in stable TSC environment, this will
cause the lapic set up a hrtimer expires after 100ms+, the restored VM now will
in stop state about 100ms+, if no other event to wake guest kernel in NO_HZ mode.

More investigation show, the MSR_IA32_TSC set from guest side has disabled TSC
synchronization in commit 0c899c25d754 (KVM: x86: do not attempt TSC
synchronization on guest writes), now host side will do TSC synchronization when
setting MSR_IA32_TSC.

I think setting MSR_IA32_TSC within 1 second from host side should not be
identified as TSC synchronization, like above case, VMM set TSC from host side
always should be updated as user want.

The MSR_IA32_TSC set code is complicated and with a long history, so I come here
to try to get help about whether my thought is correct. Here is my fix to solve
the issue, any comments are welcomed:


diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index ceb7c5e9cf9e..9380a88b9c1f 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -2722,17 +2722,6 @@ static void kvm_synchronize_tsc(struct kvm_vcpu *vcpu, u64 data)
 			 * 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;
 		}
 	}