Re: Should QEMU (accel=kvm) kvm-clock/guest_tsc stop counting during downtime blackout?

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

On 9/23/25 10:47 AM, David Woodhouse wrote:
> On Tue, 2025-09-23 at 10:25 -0700, Dongli Zhang wrote:
>>
>>
>> On 9/23/25 9:26 AM, David Woodhouse wrote:
>>> On Mon, 2025-09-22 at 12:37 -0700, Dongli Zhang wrote:
>>>> On 9/22/25 11:16 AM, David Woodhouse wrote:
>>
>> [snip]
>>
>>>>>
>>>>>>
>>>>>> As demonstrated in my test, currently guest_tsc doesn't stop counting during
>>>>>> blackout because of the lack of "MSR_IA32_TSC put" at
>>>>>> kvmclock_vm_state_change(). Per my understanding, it is a bug and we may need to
>>>>>> fix it.
>>>>>>
>>>>>> BTW, kvmclock_vm_state_change() already utilizes KVM_SET_CLOCK to re-configure
>>>>>> kvm-clock before continuing the guest VM.
>>>
>>> Yeah, right now it's probably just introducing errors for a stop/start
>>> of the VM.
>>
>> But that help can meet the expectation?
>>
>> Thanks to KVM_GET_CLOCK and KVM_SET_CLOCK, QEMU saves the clock with
>> KVM_GET_CLOCK when the VM is stopped, and restores it with KVM_SET_CLOCK when
>> the VM is continued.
> 
> It saves the actual *value* of the clock. I would prefer to phrase that
> as "it makes the clock jump backwards to the time at which the guest
> was paused".
> 
>> This ensures that the clock value itself does not change between stop and cont.
>>
>> However, QEMU does not adjust the TSC offset via MSR_IA32_TSC during stop.
>>
>> As a result, when execution resumes, the guest TSC suddenly jumps forward.
> 
> Oh wow, that seems really broken. If we're going to make it experience
> a time warp, we should at least be *consistent*.
> 
> So a guest which uses the TSC for timekeeping should be mostly
> unaffected by this and its wallclock should still be accurate. A guest
> which uses the KVM clock will be hosed by it.
> 
> I think we should fix this so that the KVM clock is unaffected too.

From my understanding of your reply, the kvm-clock/tsc should always be adjusted
whenever a QEMU VM is paused and then resumed (i.e. via stop/cont).

This applies to:

- stop / cont
- savevm / loadvm
- live migration
- cpr

It is a bug if the clock jumps backwards to the time at which the guest was paused.

The time elapsed while the VM is paused should always be accounted for and
reflected in kvm-clock/tsc once the VM resumes.

Thank you very much!

Dongli Zhang