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

[Radim Krčmář]

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").

> 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.
> 
> 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.