Re: [RFC v1 5/5] xen/arm: add SCI mediator support for DomUs

[Stefano Stabellini <sstabellini@xxxxxxxxxx>]

On Wed, 22 Dec 2021, Stefano Stabellini wrote:
> # Future Ideas
> 
> A great suggestion by Julien is to start supporting the dom0less partial
> device tree format in xl/libxl as well so that we can have a single
> "device_tree" option in dom.cfg instead of 4 (device_tree, iomem, irqs,
> dtdev).
> 
> Even with that implemented, the user has still to provide a partial dtb,
> xen,reg and xen,path. I think this is a great step forward and we should
> do that, if nothing else to make it easier to switch between dom0less
> and normal domU configurations. But the number of options and
> information that the user has to provide is still similar to what we
> have today.

I have just realized that if we start to parse the partial DTB in
xl/libxl the same way that we do for dom0less guests (parse "xen,path",
"xen,reg", and "interrupts", making dtdev, irqs and iomem optional)
actually we can achieve the goal below thanks to the combination:
"xen,path" + "xen,force-assign-without-iommu".

In other words, with dom0less we already have a way to specify the link
to the host node even if the device is not a DMA master. We can do that
by specifying both xen,path and xen,force-assign-without-iommu for a
device.

This is just FYI. I am not suggesting we should introduce dom0less-style
partial DTBs in order to get SCMI support in guests (although it would
be great to have). I think the best way forward for this series is one
of the combinations below, like a) + d), or a) + c) 


[...]

> So I think we should either:
> 
> a) extend dtdev to cover all devices, including non-DMA masters
> b) or add a new "device_assignment" property which is like dtdev but is
>    the list of both DMA masters and non-DMA masters
> 
> Either way, when non-DMA masters are present in the
> dtdev/device_assignment list we could either:
>     c) require force-assign-without-iommu="true" in dom.cfg
>     d) or print a warning like:
>     "WARNING: device assignment safety for device XXX cannot be
>     verified. Please make sure XXX is not a DMA mastering device."