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

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

* Gerd Hoffmann (kraxel@redhat.com) wrote:
>   Hi,
> 
> > > 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?
> 
> Well, good question.  I'm actually wondering what the best approach is
> to handle long-living, large buffers in virtio ...
> 
> virtio-blk (and others) are using the approach you describe.  They put a
> pointer to the io request header, followed by pointer(s) to the io
> buffers directly into the virtqueue.  That works great with storage for
> example.  The queue entries are tagged being "in" or "out" (driver to
> device or visa-versa), so the virtio transport can set up dma mappings
> accordingly or even transparently copy data if needed.
> 
> For long-living buffers where data can potentially flow both ways this
> model doesn't fit very well though.  So what virtio-gpu does instead is
> transferring the scatter list as virtio payload.  Does feel a bit
> unclean as it doesn't really fit the virtio architecture.  It assumes
> the host can directly access guest memory for example (which is usually
> the case but explicitly not required by virtio).  It also requires
> quirks in virtio-gpu to handle VIRTIO_F_IOMMU_PLATFORM properly, which
> in theory should be handled fully transparently by the virtio-pci
> transport.
> 
> We could instead have a "create-buffer" command which adds the buffer
> pointers as elements to the virtqueue as you describe.  Then simply
> continue using the buffer even after completing the "create-buffer"
> command.  Which isn't exactly clean either.  It would likewise assume
> direct access to guest memory, and it would likewise need quirks for
> VIRTIO_F_IOMMU_PLATFORM as the virtio-pci transport tears down the dma
> mappings for the virtqueue entries after command completion.
> 
> Comments, suggestions, ideas?

What about not completing the command while the device is using the
memory?

Dave

> cheers,
>   Gerd
> 
--
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK