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

[Liam Girdwood <liam.r.girdwood@linux.intel.com>]

On Tue, 2019-11-12 at 14:55 -0800, Gurchetan Singh wrote:
> On Tue, Nov 12, 2019 at 5:56 AM Liam Girdwood
> <liam.r.girdwood@linux.intel.com> wrote:
> > 
> > On Mon, 2019-11-11 at 16:54 -0800, Gurchetan Singh wrote:
> > > On Tue, Nov 5, 2019 at 2:55 AM Gerd Hoffmann <kraxel@redhat.com>
> > > wrote:
> > > > 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).
> > > 
> > > Sounds a lot like the recently added DMA_BUF_SET_NAME ioctls:
> > > 
> > > https://patchwork.freedesktop.org/patch/310349/
> > > 
> > > For virtio-wayland + virtio-vdec, the problem is sharing -- not
> > > allocation.
> > > 
> > 
> > Audio also needs to share buffers with firmware running on DSPs.
> > 
> > > As the buffer reaches a kernel boundary, it's properties devolve
> > > into
> > > [fd, size].  Userspace can typically handle sharing
> > > metadata.  The
> > > issue is the guest dma-buf fd doesn't mean anything on the host.
> > > 
> > > One scenario could be:
> > > 
> > > 1) Guest userspace (say, gralloc) allocates using virtio-
> > > gpu.  When
> > > allocating, we call uuidgen() and then pass that via
> > > RESOURCE_CREATE
> > > hypercall to the host.
> > > 2) When exporting the dma-buf, we call DMA_BUF_SET_NAME (the
> > > buffer
> > > name will be "virtgpu-buffer-${UUID}").
> > > 3) When importing, virtio-{vdec, video} reads the dma-buf name in
> > > userspace, and calls fd to handle.  The name is sent to the host
> > > via
> > > a
> > > hypercall, giving host virtio-{vdec, video} enough information to
> > > identify the buffer.
> > > 
> > > This solution is entirely userspace -- we can probably come up
> > > with
> > > something in kernel space [generate_random_uuid()] if need
> > > be.  We
> > > only need two universal IDs: {device ID, buffer ID}.
> > > 
> > 
> > I need something where I can take a guest buffer and then convert
> > it to
> > physical scatter gather page list. I can then either pass the SG
> > page
> > list to the DSP firmware (for DMAC IP programming) or have the host
> > driver program the DMAC directly using the page list (who programs
> > DMAC
> > depends on DSP architecture).
> 
> So you need the HW address space from a guest allocation? 

Yes.

>  Would your
> allocation hypercalls use something like the virtio_gpu_mem_entry
> (virtio_gpu.h) and the draft virtio_video_mem_entry (draft)?

IIUC, this looks like generic SG buffer allocation ?

> 
> struct {
>         __le64 addr;
>         __le32 length;
>         __le32 padding;
> };
> 
> /* VIRTIO_GPU_CMD_RESOURCE_ATTACH_BACKING */
> struct virtio_gpu_resource_attach_backing {
>         struct virtio_gpu_ctrl_hdr hdr;
>         __le32 resource_id;
>         __le32 nr_entries;
>       *struct struct virtio_gpu_mem_entry */
> };
> 
> struct virtio_video_mem_entry {
>     __le64 addr;
>     __le32 length;
>     __u8 padding[4];
> };
> 
> struct virtio_video_resource_attach_backing {
>     struct virtio_video_ctrl_hdr hdr;
>     __le32 resource_id;
>     __le32 nr_entries;
> };
> 
> > 
> > DSP FW has no access to userspace so we would need some additional
> > API
> > on top of DMA_BUF_SET_NAME etc to get physical hardware pages ?
> 
> The dma-buf api currently can share guest memory sg-lists.

Ok, IIUC buffers can either be shared using the GPU proposed APIs
(above) or using the dma-buf API to share via userspace ? My preference
would be to use teh more direct GPU APIs sending physical page
addresses from Guest to device driver. I guess this is your use case
too ?

Thanks

Liam

> 
> > 
> > Liam
> > 
> > 
> > 
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