Re: [PATCH V2 5/7] dt-bindings: Add xen,dev-domid property description for xen-grant DMA ops

[Oleksandr]

On 18.05.22 17:32, Arnd Bergmann wrote:


Hello Arnd


> On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@xxxxxxxxx> wrote:
> > diff --git a/Documentation/devicetree/bindings/virtio/mmio.yaml b/Documentation/devicetree/bindings/virtio/mmio.yaml
> > index 10c22b5..29a0932 100644
> > --- a/Documentation/devicetree/bindings/virtio/mmio.yaml
> > +++ b/Documentation/devicetree/bindings/virtio/mmio.yaml
> > @@ -13,6 +13,9 @@ description:
> >     See https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=virtio for
> >     more details.
> >
> > +allOf:
> > +  - $ref: /schemas/arm/xen,dev-domid.yaml#
> > +
> >   properties:
> >     compatible:
> >       const: virtio,mmio
> > @@ -33,6 +36,10 @@ properties:
> >       description: Required for devices making accesses thru an IOMMU.
> >       maxItems: 1
> >
> > +  xen,dev-domid:
> > +    description: Required when Xen grant mappings need to be enabled for device.
> > +    $ref: /schemas/types.yaml#/definitions/uint32
> > +
> >   required:
> >     - compatible
> >     - reg
> Sorry for joining the discussion late. Have you considered using the
> generic iommu
> binding here instead of a custom property?

I have to admit - no, I haven't. I was thinking that Xen specific 
feature should be communicated using Xen specific DT property.


>   This would mean having a device
> node for the grant-table mechanism that can be referred to using the 'iommus'
> phandle property, with the domid as an additional argument.

I assume, you are speaking about something like the following?


xen_dummy_iommu {
   compatible = "xen,dummy-iommu";
   #iommu-cells = <1>;
};

virtio@3000 {
   compatible = "virtio,mmio";
   reg = <0x3000 0x100>;
   interrupts = <41>;

   /* The device is located in Xen domain with ID 1 */
   iommus = <&xen_dummy_iommu 1>;
};


> It does not quite fit the model that Linux currently uses for iommus,
> as that has an allocator for dma_addr_t space

yes (# 3/7 adds grant-table based allocator)


> , but it would think it's
> conceptually close enough that it makes sense for the binding.

Interesting idea. I am wondering, do we need an extra actions for this 
to work in Linux guest (dummy IOMMU driver, etc)?


>           Arnd

--
Regards,

Oleksandr Tyshchenko