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

[Glauber Costa <gcosta@redhat.com>]

Gerd Hoffmann wrote:
> 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
> 
Agreed.

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