Re: [QEMU PATCH] kvmclock: advance clock by time window between vm_stop and pre_save

Radim Krčmář
2016-11-04T17:16:10Z

2016-11-04 16:57+0100, Paolo Bonzini:
> On 04/11/2016 16:48, Radim Krčmář wrote:
>> 2016-11-04 16:33+0100, Paolo Bonzini:
>>> On 04/11/2016 16:25, Radim Krčmář wrote:
>>>>>>
>>>>>> + if (s->advance_clock && s->clock + s->advance_clock > s->clock) {
>>>>>> + s->clock += s->advance_clock;
>>>>>> + s->advance_clock = 0;
>>>>>> + }
>>>> Can't the advance_clock added to the migrated KVMClockState instead of
>>>> passing it as another parameter?
>>>>
>>>> (It is sad that we can't just query KVMClockState in kvmclock_pre_save
>>>> because of the Linux bug.)
>>>
>>> What Linux bug? The one that makes us use kvmclock_current_nsec?
>>
>> No, the one that forced Marcelo to add the 10 minute limit to the
>> advance_clock. We wouldn't need this advance_clock hack if we could
>> just call KVM_GET_CLOCK like we did before 00f4d64ee76e ("kvmclock:
>> clock should count only if vm is running").
>
> There are two cases:
>
> - migrating a paused guest
>
> - pausing at the end of migration
>
> In the first case, kvmclock_vm_state_change's !running branch will see
> state == RUN_STATE_FINISH_MIGRATE && s->clock_valid. In the second
> case, it will see state == RUN_STATE_FINISH_MIGRATE && !s->clock_valid.

I lift my case, marcelo's said that stopping the time is a feature ...
(*kittens die*)

> In the second case it should do nothing. Then kvmclock_pre_save will
> see !s->clock_valid and call KVM_GET_CLOCK. A bonus is that, if
> migration fails and migration_thread() calls vm_start(), then the guest
> will not see the clock drift backwards.

Yes, moving KVM_GET_CLOCK to kvmclock_pre_save() and doing nothing in
kvmclock_vm_state_change() to avoid the drift on failed migration would
be nicer. We'd then also get rid of the ns migration parameter.

>>> It should work with 4.9-rc (well, once Linus applies my pull request).
>>> 4.9-rc will not return ktime_get_ns for KVM_GET_CLOCK; it will return
>>> the raw value from the kernel timekeeper.

Oh, and this does introduce a minor bug to this patch -- the time
counted by KVM_GET_CLOCK is has different frequency CLOCK_MONOTONIC.
Not accounting for that is bearable.

>>> I'm thinking that we should add a KVM capability for this, and skip
>>> kvmclock_current_nsec if the capability is present. The first part is
>>> trivial, so we can do it even during Linux rc period.
>>
>> I agree, that sounds like a nice improvement.
>
> Ok, I'll send the KVM patch then. It can even be the value *returned*
> for KVM_CAP_ADJUST_CLOCK---right now it returns 1, we can return 3 (bit
> 0 for KVM_CAP_ADJUST_CLOCK_RESENT and bit 1 for returning the raw
> value). Any suggestion for the name?

KVM_CAP_GET_CLOCK_MASTERCLOCK?

to show that the time is counted differently when using master clock.
Another idea would be KVM_CAP_GET_CLOCK_ACTUAL, because we should now
read what the guest is seeing.