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

[Juergen Gross <jgross@xxxxxxxx>]

On 19.04.22 14:17, Oleksandr wrote:
> 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
xen_is_grants_dma_device, please. Normal Xen PV devices are covered by
grants, too, and I'd like to avoid the confusion arising from this.

> > - 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?
Yes, why not.

Maybe rename xen-virtio.c to grant-dma.c?

I'd keep the XEN_VIRTIO related config option, as this will be the normal use
case. grant-dma.c should be covered by a new hidden config option XEN_GRANT_DMA
selected by XEN_VIRTIO.

CONFIG_XEN_VIRTIO should still guard xen_has_restricted_virtio_memory_access().


Juergen

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