Re: [Xen-devel] [PATCH 2/2] xen: merge temporary vcpu pinning scenarios

[Dario Faggioli <dfaggioli@xxxxxxxx>]

On Tue, 2019-07-23 at 12:42 +0000, Jan Beulich wrote:
> On 23.07.2019 11:20, Juergen Gross wrote:
> > Today there are three scenarios which are pinning vcpus temporarily
> > to
> > a single physical cpu:
> > 
> > - NMI/MCE injection into PV domains
> > - wait_event() handling
> > - vcpu_pin_override() handling
> > 
> > Each of those cases are handled independently today using their own
> > temporary cpumask to save the old affinity settings.
> 
> And what guarantees that no two of them will be in use at the same
> time?
> You don't even mention ...
> 
AFAIUI, the case of any two of the above being in use at the same time
is handled. Basically, it is fine, as far as they (temporarily) set the
affinity to the same CPU, which is said to always be the case, here...

> > The three cases can be combined as the two latter cases will only
> > pin
> > a vcpu to the physical cpu it is already running on, while
> > vcpu_pin_override() is allowed to fail.
> 
...for the first two. For vcpu_pin_override(), if it is called for
pinning the vcpu to CPU X, with any of the other twos in effect, and
having temporarily pinned the vcpu to CPU X already, then things are
fine again. If vcpu_pin_override() wants to pin to CPU Y, it fails.

All that being said, I agree it'd be good to have a bit more comments
(and I asked the same, although about a different path, in my own
reply).

Regards
-- 
Dario Faggioli, Ph.D
http://about.me/dario.faggioli
Virtualization Software Engineer
SUSE Labs, SUSE https://www.suse.com/
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