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

[Oleksandr <olekstysh@xxxxxxxxx>]

Hello Stefano, Juergen


On 18.04.22 22:11, Stefano Stabellini wrote:
> 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.

Although the main intention of this series was to enable using virtio 
devices in Xen guests, I agree that nothing in new DMA ops layer 
(xen-virtio.c) is virtio specific (at least at the moment). Regarding 
the whole patch series I am not quite sure, as it uses 
arch_has_restricted_virtio_memory_access().

>   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.
+ likely renaming everything in that patch series not to mention virtio 
(mostly related to xen-virtio.c internals).

Stefano, thank you for clarifying Christoph's point.

Well, I am not against going this direction. Could we please make a 
decision on this? @Juergen, what is your opinion?


--
Regards,

Oleksandr Tyshchenko