Re: [Xen-devel] [PATCH 2/9] vm_event: Define VM_EVENT type

[Tamas K Lengyel <tamas@xxxxxxxxxxxxx>]

On Mon, Jun 3, 2019 at 4:22 PM Tamas K Lengyel <tamas@xxxxxxxxxxxxx> wrote:
>
> > >  /* XEN_DOMCTL_mem_sharing_op.
> > > - * The CONTROL sub-domctl is used for bringup/teardown. */
> > > + * The CONTROL sub-domctl is used for bringup/teardown.
> > > + * Please note that mem sharing can be turned on *without* setting-up the
> > > + * correspondin ring
> > > + */
> >
> > As a tangent, it can? how?  (I'm entirely prepared to believe that this
> > is how the code currently works, but I can't see how such a setup would
> > plausibly work.)
>
> The vm_event ring for mem_sharing is only used to communicate
> out-of-memory condition to an external listener. I think it's only
> useful for logging since the listener wouldn't really be in a position
> to try to "make space" for the faulting domain and it would get
> destroyed after the message is sent. In any case, there isn't any
> documentation for how it was intended to be used so this is just my
> guess.

Actually, it seems the listener was intended to be able to try to
"make space" for the domain. How exactly is not clear but anyway, the
domain would get paused if there is a listener instead of just being
destroyed when there is an ENOMEM error while trying to deduplicate
shared pages.

Tamas

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