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

[Gerd Hoffmann <kraxel@redhat.com>]

Jeremy Fitzhardinge wrote:
> Gerd Hoffmann wrote:
>> I'm looking at the guest side of the issue right now, trying to identify
>> common code, and while doing so noticed that xen does the
>> version-check-loop in both get_time_values_from_xen(void) and
>> xen_clocksource_read(void), and I can't see any obvious reason for that.
>>  The loop in xen_clocksource_read(void) is not needed IMHO.  Can I
>> drop it?
> 
> No.  The get_nsec_offset() needs to be atomic with respect to the
> get_time_values() parameters.

Hmm, I somehow fail to see a case where it could be non-atomic ...

get_time_values() copies a consistent snapshot, thus
xen_clocksource_read() doesn't race against xen updating the fields.
The snapshot is in a per-cpu variable, thus it doesn't race against
other guest vcpus running get_time_values() at the same time.

> There could be a loopless
> __get_time_values() for use in this case, but given that it almost never
> loops, I don't think its worthwhile.

"in this case" ???  I'm confused.  There is only a single user of
get_nsec_offset(), which is xen_clocksource_read() ...

cheers,
  Gerd

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

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