Re: [RFC PATCH] drm/virtio: Export resource handles via DMA-buf API

From: Gerd Hoffmann
Date: Thu Sep 12 2019 - 08:38:24 EST


> To seamlessly enable buffer sharing with drivers using such frameworks,
> make the virtio-gpu driver expose the resource handle as the DMA address
> of the buffer returned from the DMA-buf mapping operation. Arguably, the
> resource handle is a kind of DMA address already, as it is the buffer
> identifier that the device needs to access the backing memory, which is
> exactly the same role a DMA address provides for native devices.

No. A scatter list has guest dma addresses, period. Stuffing something
else into a scatterlist is asking for trouble, things will go seriously
wrong when someone tries to use such a fake scatterlist as real scatterlist.

Also note that "the DMA address of the buffer" is bonkers in virtio-gpu
context. virtio-gpu resources are not required to be physically
contigous in memory, so typically you actually need a scatter list to
describe them.