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 I found https://static.linaro.org/connect/lvc21/presentations/lvc21-314.pdf but I think it's a bit out of date; the decision about how to handle grant mappings for virtio devices is still 'TBD'.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