Re: [QEMU][PATCHv2 0/8] Xen: support grant mappings.

[David Woodhouse]

On Wed, 2023-10-25 at 14:24 -0700, Vikram Garhwal wrote:
> Hi,
> This patch series add support for grant mappings as a pseudo RAM region for 
> Xen.
> 
> Enabling grant mappings patches(first 6) are written by Juergen in 2021.
> 
> QEMU Virtio device provides an emulated backends for Virtio frontned devices
> in Xen.
> Please set "iommu_platform=on" option when invoking QEMU. As this will set
> VIRTIO_F_ACCESS_PLATFORM feature which will be used by virtio frontend in Xen
> to know whether backend supports grants or not.

I don't really understand what's going on here. The subject of the
cover letter certainly doesn't help me, because we *already* support
grant mappings under Xen, don't we?

I found
https://static.linaro.org/connect/lvc21/presentations/lvc21-314.pdf but
I think it's a bit out of date; the decision about how to handle grant
mappings for virtio devices is still 'TBD'.

Can you talk me through the process of what happens when a guest wants
to a virtio device to initiate 'DMA' to one of its pages? I assume it
starts by creating a grant mapping, and then taking the gntref and...
then what?

I don't see any changes to the virtio devices themselves in this
series; are we doing something that will make it work by magic? If so,
it might be useful to explain that magic...

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