Re: pv clock: kvm is incompatible with xen :-(

[Gerd Hoffmann <kraxel@redhat.com>]

Glauber Costa wrote:
> Gerd Hoffmann wrote:
>> Jeremy Fitzhardinge wrote:
>>> Xen could change the parameters in the instant after
>>> get_time_values(). That change could be as a result of
>>> suspend-resume, so the parameters
>>> and the tsc could be wildly different.
>>
>> Ah, ok, forgot the rdtsc in the picture.  With that in mind I fully
>> agree that the loop is needed.  I think kvm guests can even hit that one
>> with the vcpu migrating to a different physical cpu, so we better handle
>> it correctly ;)
> 
> It's probably not needed for kvm, since we update everything everytime
> we get scheduled in the host side, which would cover the case for
> migration between physical cpus. 

No, it wouldn't.  The corner case we must catch is: guest reads time
info, kvm reschedules the guest to another pcpu, guest reads the tsc.
The time info used by the guest for the tsc delta is stale then, it
belongs to the previous pcpu.

cheers,
  Gerd

-- 
http://kraxel.fedorapeople.org/xenner/

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