Re: [virtio-dev] Re: Fwd: Qemu Support for Virtio Video V4L2 driver

[Alexandre Courbot <acourbot@chromium.org>]

On Thu, May 21, 2020 at 1:21 AM Nicolas Dufresne <nicolas@ndufresne.ca> wrote:
>
> Le mercredi 20 mai 2020 à 12:19 +0900, Alexandre Courbot a écrit :
> > On Wed, May 20, 2020 at 2:29 AM Nicolas Dufresne <nicolas@ndufresne.ca> wrote:
> > > Le mardi 19 mai 2020 à 17:37 +0900, Keiichi Watanabe a écrit :
> > > > Hi Nicolas,
> > > >
> > > > On Fri, May 15, 2020 at 8:38 AM Nicolas Dufresne <
> > > > nicolas@ndufresne.ca
> > > > > wrote:
> > > > > Le lundi 11 mai 2020 à 20:49 +0900, Keiichi Watanabe a écrit :
> > > > > > Hi,
> > > > > >
> > > > > > Thanks Saket for your feedback. As Dmitry mentioned, we're focusing on
> > > > > > video encoding and decoding, not camera. So, my reply was about how to
> > > > > > implement paravirtualized video codec devices.
> > > > > >
> > > > > > On Mon, May 11, 2020 at 8:25 PM Dmitry Sepp <
> > > > > > dmitry.sepp@opensynergy.com
> > > > > > wrote:
> > > > > > > Hi Saket,
> > > > > > >
> > > > > > > On Montag, 11. Mai 2020 13:05:53 CEST Saket Sinha wrote:
> > > > > > > > Hi Keiichi,
> > > > > > > >
> > > > > > > > I do not support the approach of  QEMU implementation forwarding
> > > > > > > > requests to the host's vicodec module since  this can limit the scope
> > > > > > > > of the virtio-video device only for testing,
> > > > > > >
> > > > > > > That was my understanding as well.
> > > > > >
> > > > > > Not really because the API which the vicodec provides is V4L2 stateful
> > > > > > decoder interface [1], which are also used by other video drivers on
> > > > > > Linux.
> > > > > > The difference between vicodec and actual device drivers is that
> > > > > > vicodec performs decoding in the kernel space without using special
> > > > > > video devices. In other words, vicodec is a software decoder in kernel
> > > > > > space which provides the same interface with actual video drivers.
> > > > > > Thus, if the QEMU implementation can forward virtio-video requests to
> > > > > > vicodec, it can forward them to the actual V4L2 video decoder devices
> > > > > > as well and VM gets access to a paravirtualized video device.
> > > > > >
> > > > > > The reason why we discussed vicodec in the previous thread was it'll
> > > > > > allow us to test the virtio-video driver without hardware requirement.
> > > > > >
> > > > > > [1]
> > > > > > https://www.kernel.org/doc/html/latest/media/uapi/v4l/dev-decoder.html
> > > > > >
> > > > > >
> > > > > > > > which instead can be used with multiple use cases such as -
> > > > > > > >
> > > > > > > > 1. VM gets access to paravirtualized  camera devices which shares the
> > > > > > > > video frames input through actual HW camera attached to Host.
> > > > > > >
> > > > > > > This use-case is out of the scope of virtio-video. Initially I had a plan to
> > > > > > > support capture-only streams like camera as well, but later the decision was
> > > > > > > made upstream that camera should be implemented as separate device type. We
> > > > > > > still plan to implement a simple frame capture capability as a downstream
> > > > > > > patch though.
> > > > > > >
> > > > > > > > 2. If Host has multiple video devices (especially in ARM SOCs over
> > > > > > > > MIPI interfaces or USB), different VM can be started or hotplugged
> > > > > > > > with selective video streams from actual HW video devices.
> > > > > > >
> > > > > > > We do support this in our device implementation. But spec in general has no
> > > > > > > requirements or instructions regarding this. And it is in fact flexible
> > > > > > > enough
> > > > > > > to provide abstraction on top of several HW devices.
> > > > > > >
> > > > > > > > Also instead of using libraries like Gstreamer in Host userspace, they
> > > > > > > > can also be used inside the VM userspace after getting access to
> > > > > > > > paravirtualized HW camera devices .
> > > > > >
> > > > > > Regarding Gstreamer, I intended this video decoding API [2]. If QEMU
> > > > > > can translate virtio-video requests to this API, we can easily support
> > > > > > multiple platforms.
> > > > > > I'm not sure how feasible it is though, as I have no experience of
> > > > > > using this API by myself...
> > > > >
> > > > > Not sure which API you aim exactly, but what one need to remember is that
> > > > > mapping virtio-video CODEC on top of VAAPI, V4L2 Stateless, NVDEC or other type
> > > > > of "stateless" CODEC is not trivial and can't be done without userspace. Notably
> > > > > because we don't want to do bitstream parsing in the kernel on the main CPU as
> > > > > security would otherwise be very hard to guaranty. The other driver using same
> > > > > API as virtio-video do bitstream parsing on a dedicated co-processor (through
> > > > > firmware blobs though).
> > > > >
> > > > > Having bridges between virtio-video, qemu and some abstraction library like
> > > > > FFMPEG or GStreamer is certainly the best solution if you want to virtualize any
> > > > > type of HW accelerated decoder or if you need to virtualized something
> > > > > proprietary (like NVDEC). Please shout if you need help.
> > > > >
> > > >
> > > > Yeah, I meant we should map virtio-video commands to a set of
> > > > abstracted userspace APIs to avoid having many platform-dependent code
> > > > in QEMU.
> > > > This is the same with what we implemented in crosvm, a VMM on
> > > > ChromiumOS. Crosvm's video device translates virtio-video commands
> > > > into our own video decoding APIs [1, 2] which supports VAAPI, V4L2
> > > > stateful and V4L2 stateless. Unfortunately, since our library is
> > > > highly depending on Chrome, we cannot reuse this for QEMU.
> > > >
> > > > So, I agree that using FFMPEG or GStreamer is a good idea. Probably,
> > > > APIs in my previous link weren't for this purpose.
> > > > Nicolas, do you know any good references for FFMPEG or GStreamer's
> > > > abstracted video decoding APIs? Then, I may be able to think about how
> > > > virtio-video protocols can be mapped to them.
> > >
> > > The FFMpeg API for libavcodec can be found here:
> > >
> > >   http://git.videolan.org/?p=ffmpeg.git;a=blob;f=libavcodec/avcodec.h
> > >
> > > GStreamer does not really have such a low level CODEC API. So while
> > > it's possible to use it (Wine project uses it for it's parsers as an
> > > example, and Firefox use to have CODEC support wrapping GStreamer
> > > CODEC), there will not be any one-to-one mapping. GStreamer is often
> > > chosen as it's LGPL code does not carry directly any patented
> > > implementation. It instead rely on plugins, which maybe provided as
> > > third party, allowing to distribute your project while giving uses the
> > > option to install potentially non-free technologies.
> > >
> > > But overall, I can describe GStreamer API for CODEC wrapping (pipeline
> > > less) as:
> > >
> > >   - Push GstCaps describing the stream format
> > >   - Push bitstream buffer on sink pad
> > >   - When ready, buffers will be pushed through the push function
> > >     callback on src pad
> > >
> > > Of course nothing prevent adding something like the vda abstraction in
> > > qemu and make this multi-backend capable.
> >
> > My understanding is that we don't need a particularly low-level API to
> > interact with. The host virtual device is receiving the whole encoded
> > data, and can thus easily reconstruct the original stream (minus the
> > container) and pass it to ffmpeg/gstreamer. So we can be pretty
> > high-level here.
> >
> > Now the choice of API will also determine whether we want to allow
> > emulation of codec devices, or whether we stay on a purely
> > para-virtual track. If we use e.g. gstreamer, then the host can
> > provide a virtual device that is backed by a purely software
> > implementation. This can be useful for testing purposes, but for
> > real-life usage the guest would be just as well using gstreamer
> > itself.
>
> Agreed.
>
> >
> > If we want to make sure that there is hardware on the host side, then
> > an API like libva might make more sense, but it would be more
> > complicated and may not support all hardware (I don't know if the V4L2
> > backends are usable for instance).
>
> To bring VAAPI into Qemu directly you'd have to introduce bitstream
> parser, DPB management and other CODEC specific bits. I cannot speak
> for the project, but that's re-inventing the wheel again with very
> little gain. Best is to open the discussion with them early.
>
> Note that it's relatively simple in both framework to only choose HW
> accelerated CODECs. In ffmpeg, HW accelerator codecs can only be used
> with HWContext, so your wrapper need to know specific HWContext for the
> specific accelerator. In GStreamer, since 1.16, we add a metadata that
> let the user know which decoder is hardware accelerated. (This is
> usually used to disable HW acceleration at the moment).

Good point, and that would also not close the door to exposing a
software-backed device for testing purposes.