linux-kernel - Re: [RFC PATCH 6/6] arm/xen: Assign xen-virtio DMA ops for virtio devices in Xen guests

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Date:   Mon, 18 Apr 2022 12:11:00 -0700 (PDT)
From:   Stefano Stabellini <sstabellini@...nel.org>
To:     Oleksandr <olekstysh@...il.com>
cc:     Christoph Hellwig <hch@...radead.org>,
        Stefano Stabellini <sstabellini@...nel.org>,
        xen-devel@...ts.xenproject.org, linux-kernel@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org,
        Oleksandr Tyshchenko <oleksandr_tyshchenko@...m.com>,
        Boris Ostrovsky <boris.ostrovsky@...cle.com>,
        Juergen Gross <jgross@...e.com>, Julien Grall <julien@....org>,
        "Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [RFC PATCH 6/6] arm/xen: Assign xen-virtio DMA ops for virtio
 devices in Xen guests

On Mon, 18 Apr 2022, Oleksandr wrote:
> On 16.04.22 09:07, Christoph Hellwig wrote:
> 
> Hello Christoph
> 
> > On Fri, Apr 15, 2022 at 03:02:45PM -0700, Stefano Stabellini wrote:
> > > This makes sense overall. Considering that the swiotlb-xen case and the
> > > virtio case are mutually exclusive, I would write it like this:
> > Curious question:  Why can't the same grant scheme also be used for
> > non-virtio devices?  I really hate having virtio hooks in the arch
> > dma code.  Why can't Xen just say in DT/ACPI that grants can be used
> > for a given device?

[...]

> This patch series tries to make things work with "virtio" devices in Xen
> system without introducing any modifications to code under drivers/virtio.


Actually, I think Christoph has a point.

There is nothing inherently virtio specific in this patch series or in
the "xen,dev-domid" device tree binding. Assuming a given device is
emulated by a Xen backend, it could be used with grants as well.

For instance, we could provide an emulated e1000 NIC with a
"xen,dev-domid" property in device tree. Linux could use grants with it
and the backend could map the grants. It would work the same way as
virtio-net/block/etc. Passthrough devices wouldn't have the
"xen,dev-domid" property, so no problems.

So I think we could easily generalize this work and expand it to any
device. We just need to hook on the "xen,dev-domid" device tree
property.

I think it is just a matter of:
- remove the "virtio,mmio" check from xen_is_virtio_device
- rename xen_is_virtio_device to something more generic, like
  xen_is_grants_device
- rename xen_virtio_setup_dma_ops to something more generic, like
  xen_grants_setup_dma_ops

And that's pretty much it.