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

[Stefano Stabellini]

On Thu, 26 Oct 2023, David Woodhouse wrote:
> 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 I think it's a bit out of date; the decision about how to handle grant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'.

See this presentation:
https://www.youtube.com/watch?v=boRQ8UHc760

The patch series is for the guest (e.g. Linux) to use grants to share
memory with virtio devices. The plumbing was already done in Linux a
couple of years ago, but QEMU support for it is still missing.


> 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?

First the guest gets a grant reference for the page, then it uses it as
"dma address" to pass to the virtio device. The top bit (1ULL << 63) is
used to signal that the address in question is a grant address.

See in Linux:
drivers/xen/grant-dma-ops.c grant_to_dma, dma_to_grant,
xen_grant_dma_alloc, etc.


> 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...

Yes something like that :-)
https://marc.info/?l=xen-devel&m=165419780304962&w=2

Vikram, I think it would be a good idea to submit a separate patch to
xen.git to add a document under docs/ to capture this.