Re: [virtio-dev] [PATCH] [RFC RESEND] vdec: Add virtio video decode device specification

[Gerd Hoffmann <kraxel@redhat.com>]

  Hi,

> Our prototype implementation uses [4], which allows the virtio-vdec
> device to use buffers allocated by virtio-gpu device.

> [4] https://lkml.org/lkml/2019/9/12/157

Well.  I think before even discussing the protocol details we need a
reasonable plan for buffer handling.  I think using virtio-gpu buffers
should be an optional optimization and not a requirement.  Also the
motivation for that should be clear (Let the host decoder write directly
to virtio-gpu resources, to display video without copying around the
decoded framebuffers from one device to another).

Referencing virtio-gpu buffers needs a better plan than just re-using
virtio-gpu resource handles.  The handles are device-specific.  What if
there are multiple virtio-gpu devices present in the guest?

I think we need a framework for cross-device buffer sharing.  One
possible option would be to have some kind of buffer registry, where
buffers can be registered for cross-device sharing and get a unique
id (a uuid maybe?).  Drivers would typically register buffers on
dma-buf export.

Another option would be to pass around both buffer handle and buffer
owner, i.e. instead of "u32 handle" have something like this:

struct buffer_reference {
	enum device_type; /* pci, virtio-mmio, ... */
	union device_address {
		struct pci_address pci_addr;
		u64 virtio_mmio_addr;
		[ ... ]
	};
	u64 device_buffer_handle; /* device-specific, virtio-gpu could use resource ids here */
};

cheers,
  Gerd