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

From: Mirela Simonovic <mirela.simonovic@aggios.com>
To: Julien Grall <julien.grall@arm.com>
Cc: Stefano Stabellini <sstabellini@kernel.org>,
	Xen Devel <xen-devel@lists.xen.org>,
	Davorin Mista <dm@aggios.com>,
	Saeed Nowshadi <saeed.nowshadi@xilinx.com>,
	xen-devel <xen-devel@lists.xenproject.org>,
	Stefano Stabellini <stefano.stabellini@xilinx.com>
Subject: Re: [PATCH 17/18] xen/arm: Resume Dom0 after Xen resumes
Date: Fri, 16 Nov 2018 13:34:21 +0100	[thread overview]
Message-ID: <CAKPH-Ngn1vp0oxtXs_cO5WnCJJExg5y=QQO5tck6fd1Z76ozsg@mail.gmail.com> (raw)
In-Reply-To: <dc9b6382-490e-fd41-6fec-faf211103cee@arm.com>

Hi Julien,

On Fri, Nov 16, 2018 at 12:44 PM Julien Grall <julien.grall@arm.com> wrote:
>
>
>
> On 16/11/2018 11:29, Mirela Simonovic wrote:
> > On Fri, Nov 16, 2018 at 11:33 AM Mirela Simonovic
> > <mirela.simonovic@aggios.com> wrote:
> >>
> >> Hi Julien,
> >>
> >> On Thu, Nov 15, 2018 at 9:31 PM Julien Grall <julien.grall@arm.com> 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.

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.
If this assumption is correct, I strongly suggest to remove this patch
and let the wake-up for all domains happen automatically after the Xen
resumes.

> Cheers,
>
> --
> Julien Grall

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