guest / host buffer sharing ...

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi folks,

The issue of sharing buffers between guests and hosts keeps poping
up again and again in different contexts.  Most recently here:

https://www.mail-archive.com/qemu-devel@nongnu.org/msg656685.html

So, I'm grabbing the recipient list of the virtio-vdec thread and some
more people I know might be interested in this, hoping to have everyone
included.

Reason is:  Meanwhile I'm wondering whenever "just use virtio-gpu
resources" is really a good answer for all the different use cases
we have collected over time.  Maybe it is better to have a dedicated
buffer sharing virtio device?  Here is the rough idea:


(1) The virtio device
=====================

Has a single virtio queue, so the guest can send commands to register
and unregister buffers.  Buffers are allocated in guest ram.  Each buffer
has a list of memory ranges for the data.  Each buffer also has some
properties to carry metadata, some fixed (id, size, application), but
also allow free form (name = value, framebuffers would have
width/height/stride/format for example).


(2) The linux guest implementation
==================================

I guess I'd try to make it a drm driver, so we can re-use drm
infrastructure (shmem helpers for example).  Buffers are dumb drm
buffers.  dma-buf import and export is supported (shmem helpers
get us that for free).  Some device-specific ioctls to get/set
properties and to register/unregister the buffers on the host.


(3) The qemu host implementation
================================

qemu (likewise other vmms) can use the udmabuf driver to create
host-side dma-bufs for the buffers.  The dma-bufs can be passed to
anyone interested, inside and outside qemu.  We'll need some protocol
for communication between qemu and external users interested in those
buffers, to receive dma-bufs (via unix file descriptor passing) and
update notifications.  Dispatching updates could be done based on the
application property, which could be "virtio-vdec" or "wayland-proxy"
for example.


commments?

cheers,
  Gerd