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

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Message-ID: <20190912123821.rraib5entkcxt5p5@sirius.home.kraxel.org>
Date:   Thu, 12 Sep 2019 14:38:21 +0200
From:   Gerd Hoffmann <kraxel@...hat.com>
To:     Tomasz Figa <tfiga@...omium.org>
Cc:     David Airlie <airlied@...ux.ie>, Daniel Vetter <daniel@...ll.ch>,
        dri-devel@...ts.freedesktop.org,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, stevensd@...omium.org,
        marcheu@...omium.org, zachr@...omium.org, keiichiw@...omium.org,
        posciak@...omium.org
Subject: Re: [RFC PATCH] drm/virtio: Export resource handles via DMA-buf API

  Hi,

> 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.

cheers,
  Gerd