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

[Gurchetan Singh <gurchetansingh@chromium.org>]

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

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.

>
> Liam
>
>
>
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