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

[Arnd Bergmann <arnd@xxxxxxxx>]

On Wed, May 18, 2022 at 5:06 PM Oleksandr <olekstysh@xxxxxxxxx> wrote:
> On 18.05.22 17:32, Arnd Bergmann wrote:
> > On Sat, May 7, 2022 at 7:19 PM Oleksandr Tyshchenko <olekstysh@xxxxxxxxx> 
> > wrote:
>
> >   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>;
> };

Right, that's that's the idea, except I would not call it a 'dummy'.
>From the perspective of the DT, this behaves just like an IOMMU,
even if the exact mechanism is different from most hardware IOMMU
implementations.

> > 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)?

It depends on how closely the guest implementation can be made to
resemble a normal iommu. If you do allocate dma_addr_t addresses,
it may actually be close enough that you can just turn the grant-table
code into a normal iommu driver and change nothing else.

        Arnd