On Thu, 25 Apr 2024 at 13:24, Michael S. Tsirkin <[email protected]> wrote: > > On Thu, Apr 25, 2024 at 01:04:31PM +0300, Manos Pitsidianakis wrote: > > On Thu, 25 Apr 2024 at 10:49, Mark Cave-Ayland > > <[email protected]> wrote: > > > > > > On 25/04/2024 07:30, Manos Pitsidianakis wrote: > > > > > > > On Wed, 24 Apr 2024 at 13:31, Mark Cave-Ayland > > > > <[email protected]> wrote: > > > >> > > > >> On 23/04/2024 12:05, Philippe Mathieu-Daudé wrote: > > > >> > > > >>> On 23/4/24 11:18, Manos Pitsidianakis wrote: > > > >>>> On Tue, 23 Apr 2024 at 11:47, Manos Pitsidianakis > > > >>>> <[email protected]> wrote: > > > >>>>> > > > >>>>> On Tue, 23 Apr 2024 at 00:11, Michael S. Tsirkin <[email protected]> > > > >>>>> wrote: > > > >>>>>> > > > >>>>>> On Mon, Apr 22, 2024 at 11:07:21PM +0200, Philippe Mathieu-Daudé > > > >>>>>> wrote: > > > >>>>>>> On 22/4/24 23:02, Michael S. Tsirkin wrote: > > > >>>>>>>> On Mon, Apr 22, 2024 at 04:20:56PM +0200, Philippe Mathieu-Daudé > > > >>>>>>>> wrote: > > > >>>>>>>>> Since VirtIO devices can change endianness at runtime, > > > >>>>>>>>> we need to use the device endianness, not the target > > > >>>>>>>>> one. > > > >>>>>>>>> > > > >>>>>>>>> Cc: [email protected] > > > >>>>>>>>> Fixes: eb9ad377bb ("virtio-sound: handle control messages and > > > >>>>>>>>> streams") > > > >>>>>>>>> Signed-off-by: Philippe Mathieu-Daudé <[email protected]> > > > >>>>>>>> > > > >>>>>>>> > > > >>>>>>>> > > > >>>>>>>> This is all completely bogus. Virtio SND is from Virtio 1.0 only. > > > >>>>>>>> It is unconditionally little endian. > > > >>>>> > > > >>>>> > > > >>>>> This part of the code is for PCM frames (raw bytes), not virtio spec > > > >>>>> fields (which indeed must be LE in modern VIRTIO). > > > >>>> > > > >>>> Thought a little more about it. We should keep the target's > > > >>>> endianness > > > >>>> here, if it's mutable then we should query the machine the device is > > > >>>> attached to somehow. the virtio device should never change endianness > > > >>>> like Michael says since it's not legacy. > > > >>> > > > >>> Grr. So as Richard suggested, this need to be pass as a device > > > >>> property then. > > > >>> (https://lore.kernel.org/qemu-devel/[email protected]/) > > > >> > > > >> It feels to me that the endianness is something that should be > > > >> negotiated as part of > > > >> the frame format, since the endianness of the audio hardware can be > > > >> different from > > > >> that of the CPU (think PReP machines where it was common that a big > > > >> endian CPU is > > > >> driving little endian hardware as found on x86). > > > > > > > > But that is the job of the hardware drivers, isn't it? Here we are > > > > taking frames passed from the guest to its virtio driver in the format > > > > specified in the target cpu's endianness and QEMU as the device passes > > > > it to host ALSA/Pipewire/etc which in turn passes it to the actual > > > > audio hardware driver.. > > > > > > The problem is that the notion of target CPU endian is not fixed. For > > > example the > > > PowerPC CPU starts off in big-endian mode, but these days most systems > > > will switch > > > the CPU to little-endian mode on startup to run ppc64le. There's also the > > > ILE bit > > > which can be configured so that a big-endian PowerPC CPU can dynamically > > > switch to > > > little-endian mode when processing an interrupt, so you could potentially > > > end up with > > > either depending upon the current mode of the CPU. > > > > > > These are the kinds of issues that led to the later virtio specifications > > > simply > > > using little-endian for everything, since then there is zero ambiguity > > > over what > > > endian is required for the virtio configuration space accesses. > > > > > > It feels to me that assuming a target CPU endian is fixed for the PCM > > > frame formats > > > is simply repeating the mistakes of the past - and even the fact that we > > > are > > > discussing this within this thread suggests that at a very minimum the > > > virtio-snd > > > specification needs to be updated to clarify the byte ordering of the PCM > > > frame formats. > > > > > > > > > ATB, > > > > > > Mark. > > > > > > > > > Agreed, I think we are saying approximately the same thing here. > > > > We need a mechanism to retrieve the vCPUs endianness and a way to > > notify subscribed devices when it changes. > > I don't think I agree, it's not the same thing. > Guest should just convert and send data in LE format. > Host should then convert from LE format. > Target endian-ness does not come into it.
That's not in the VIRTIO 1.2 spec. We are talking about supporting things as they currently stand, not as they could have been.
