linux-kernel - Re: [PATCH 2/2] xen/virtio: Avoid use of the dom0 backend in dom0

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Message-ID: <ZKPO5WbFGblXU5hX@MacBook-Air-de-Roger.local>
Date:   Tue, 4 Jul 2023 09:48:53 +0200
From:   Roger Pau Monné <roger.pau@...rix.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     Oleksandr Tyshchenko <Oleksandr_Tyshchenko@...m.com>,
        Petr Pavlu <petr.pavlu@...e.com>,
        "xen-devel@...ts.xenproject.org" <xen-devel@...ts.xenproject.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jgross@...e.com" <jgross@...e.com>, vikram.garhwal@....com
Subject: Re: [PATCH 2/2] xen/virtio: Avoid use of the dom0 backend in dom0

On Thu, Jun 29, 2023 at 03:44:04PM -0700, Stefano Stabellini wrote:
> On Thu, 29 Jun 2023, Oleksandr Tyshchenko wrote:
> > On 29.06.23 04:00, Stefano Stabellini wrote:
> > > I think we need to add a second way? It could be anything that can help
> > > us distinguish between a non-grants-capable virtio backend and a
> > > grants-capable virtio backend, such as:
> > > - a string on xenstore
> > > - a xen param
> > > - a special PCI configuration register value
> > > - something in the ACPI tables
> > > - the QEMU machine type
> > 
> > 
> > Yes, I remember there was a discussion regarding that. The point is to 
> > choose a solution to be functional for both PV and HVM *and* to be able 
> > to support a hotplug. IIRC, the xenstore could be a possible candidate.
> 
> xenstore would be among the easiest to make work. The only downside is
> the dependency on xenstore which otherwise virtio+grants doesn't have.

I would avoid introducing a dependency on xenstore, if nothing else we
know it's a performance bottleneck.

We would also need to map the virtio device topology into xenstore, so
that we can pass different options for each device.

> Vikram is working on virtio with grants support in QEMU as we speak.
> Maybe we could find a way to add a flag in QEMU so that we can detect at
> runtime if a given virtio device support grants or not.

Isn't there a way for the device to expose capabilities already?  For
example how does a virtio-blk backend expose support for indirect
descriptors?

Thanks, Roger.