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

[Juergen Gross]

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.

Stefano, thank you for clarifying Christoph's point.

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

OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

OpenPGP_signature
Description: OpenPGP digital signature