Re: Fwd: VirtioSound device emulation implementation

[Shreyansh Chouhan <chouhan.shreyansh2702@gmail.com>]

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On Thu, 28 Jan 2021 at 16:54, Alex Bennée <alex.bennee@linaro.org> wrote:

>
> Shreyansh Chouhan <chouhan.shreyansh2702@gmail.com> writes:
>
> > Thanks a lot Alex!
> >
> >> All QEMU devices have two parts, a frontend (which the guest sees) and a
> >> backend (which is how the data gets to somewhere in the host). Some of
> >> the command line options in QEMU elide the details for convenience (-nic
> >> and -drive are examples). The -netdev device is all about configuring
> >> the backend of the network part for a paired -device front end. There is
> >> a similar setup for audio (-audiodev) although I'll defer to Gerd's
> >> expertise on how the two interact.
> >
> > This helped me understand a bit more about how the devices work. In the
> > source
> > code, I found the `virtio-net-pci.c` and `virtio-net.c` files, I think
> the
> > pci device is what is visible to the guest.
> > So `virtio-net-pci.c` should be the front end?
>
> No - they are both front-ends. In VirtIO you have two transports. The
> first is virtio-mmio which is a simple set of memory mapped IO
> registers. There is no discovery (well there is some but the guest needs
> to find where in memory the mmio bus is via some method like hardcoded
> address or DTB).
>
> The second is virtio-pci. Here the virtio devices are encapsulated in a
> PCI bus so you get the benefits of discovery and bus enumeration.
>
So I am a bit confused here. First of there are two parts to a virtio
device in general. One is the
front end that the guest sees, and one is where the processing happens,
which could be either in QEMU
or as a separate daemon. In case of an in QEMU virtio device, the QEMU
front end is replaced by the
virtio-front end. Is that true? That's what I think should happen if both
of those files are front ends.

When you said earlier that all QEMU devices have two parts, I somehow
thought that
the in-QEMU backend for the audio device will itself have two parts.

>
> > For the realize function, I saw that the `virtio_net_device_realize`
> > function initializes
> > the `net_conf` for the device. So I am guessing that the
> > `virtio_snd_device_realize` function
> > should initialize the number of jacks and streams a device has, along
> with
> > the configuration
> > for all these jacks and streams?
>
> No those are all front-end configuration knobs.
>
I am assuming these are set from the command-line or the configuration
files.

>
> > The thing is I do not understand `net` devices all that well so I get a
> bit
> > confused with
> > what is specific to a net device and what should I actually be worried
> > about :)
> >
> > The device initalization step mentions that the jack and streams should
> be
> > read and
> > a query should be made for the config of all jacks and streams. What
> should
> > be the
> > default values of these configurations? I think the realize function is
> > responsible
> > for setting these up.
>
> Gerd?
>
>
>
> --
> Alex Bennée
>
--
Shreyansh

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