Re: [Xen-devel] [PATCH 17/18] xen/arm: Resume Dom0 after Xen resumes

[Julien Grall <julien.grall@xxxxxxx>]

On 16/11/2018 12:34, Mirela Simonovic wrote:
> Hi Julien,
Hi Mirela,

> On Fri, Nov 16, 2018 at 12:44 PM Julien Grall <julien.grall@xxxxxxx> wrote:
> > On 16/11/2018 11:29, Mirela Simonovic wrote:
> > > On Fri, Nov 16, 2018 at 11:33 AM Mirela Simonovic
> > > <mirela.simonovic@xxxxxxxxxx> wrote:
> > > > Hi Julien,
> > > >
> > > > On Thu, Nov 15, 2018 at 9:31 PM Julien Grall <julien.grall@xxxxxxx> wrote:
> > > > > Hi,
> > > > >
> > > > > On 11/12/18 11:30 AM, Mirela Simonovic wrote:
> > > > > > The resume of Dom0 should always follow Xen's resume. This is
> > > > > > done by unblocking the first vCPU of Dom0.
> > > > > Please explain why you always need to resume Dom0 afterwards.
> > > > We don't need to, but that is what is promised in the design spec.
> > You surely had some rationale when writing the promise in the design document,
> > right?
> >
> > So what is the reason behind it? I don't want to resume a domain if that's not
> > necessary.
> >
> > > >
> > > To be more specific - a domU that doesn't depend on dom0 can resume
> > > and work happily without dom0 being resumed, i.e. just Xen and domU
> > > resume. So patch "[PATCH 17/18] xen/arm: Resume Dom0 after Xen
> > > resumes" is not a must (when there are no PV drivers involved).
> > PV backends don't necessarily reside in the hardware domain. So how is this
> > going to work for the other case?
> I honestly believe that this is not necessary, and is sub-optimal. It
> relies on an assumption that dom0 contains all the PV drivers, which
> is not always correct.
Well, there are other reasons to resume the hardware domain. The hardware domain 
owns most the devices and may be part of the suspend/resume path.
As you tie the host suspend to the hardware domain suspend, it may makes sense 
to resume at the same time. It is the kind of rationale I would expect in the 
commit message.
> I would prefer if someone can tell us that any frontend will trigger
> an event to the backend, and the event would go through Xen. That way,
> this event would cause a domain containing the backend driver to
> resume. I think this is the best possible solution, but it relies on
> an assumption that the event will go through Xen, and I'm not
> knowledgeable enough to claim that this is indeed the case.
I think it is should work, the best way to find out if building a test case for 
it.

Cheers,

--
Julien Grall

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