Re: [virtio-dev] Re: guest / host buffer sharing ...

["Dr. David Alan Gilbert" <dgilbert@redhat.com>]

* Gerd Hoffmann (kraxel@redhat.com) wrote:
>   Hi,
> 
> > > 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:
> > 
> > My concern is that buffer sharing isn't a "device".  It's a primitive
> > used in building other devices.  When someone asks for just buffer
> > sharing it's often because they do not intend to upstream a
> > specification for their device.
> 
> Well, "vsock" isn't a classic device (aka nic/storage/gpu/...) either.
> It is more a service to allow communication between host and guest
> 
> That buffer sharing device falls into the same category.  Maybe it even
> makes sense to build that as virtio-vsock extension.  Not sure how well
> that would work with the multi-transport architecture of vsock though.
> 
> > If this buffer sharing device's main purpose is for building proprietary
> > devices without contributing to VIRTIO, then I don't think it makes
> > sense for the VIRTIO community to assist in its development.
> 
> One possible use case would be building a wayland proxy, using vsock for
> the wayland protocol messages and virtio-buffers for the shared buffers
> (wayland client window content).
> 
> It could also simplify buffer sharing between devices (feed decoded
> video frames from decoder to gpu), although in that case it is less
> clear that it'll actually simplify things because virtio-gpu is
> involved anyway.
> 
> We can't prevent people from using that for proprietary stuff (same goes
> for vsock).
> 
> There is the option to use virtio-gpu instead, i.e. add support to qemu
> to export dma-buf handles for virtio-gpu resources to other processes
> (such as a wayland proxy).  That would provide very similar
> functionality (and thereby create the same loophole).
> 
> > VIRTIO recently gained a shared memory resource concept for access to
> > host memory.  It is being used in virtio-pmem and virtio-fs (and
> > virtio-gpu?).
> 
> virtio-gpu is in progress still unfortunately (all kinds of fixes for
> the qemu drm drivers and virtio-gpu guest driver refactoring kept me
> busy for quite a while ...).
> 
> > If another flavor of shared memory is required it can be
> > added to the spec and new VIRTIO device types can use it.  But it's not
> > clear why this should be its own device.
> 
> This is not about host memory, buffers are in guest ram, everything else
> would make sharing those buffers between drivers inside the guest (as
> dma-buf) quite difficult.

Given it's just guest memory, can the guest just have a virt queue on
which it places pointers to the memory it wants to share as elements in
the queue?

Dave

> > My question would be "what is the actual problem you are trying to
> > solve?".
> 
> Typical use cases center around sharing graphics data between guest
> and host.
> 
> cheers,
>   Gerd
> 
> 
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--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK