Re: [RFC PATCH 1/1] virtio: write back features before verify

[Michael S. Tsirkin]

On Tue, Oct 05, 2021 at 09:25:39AM +0200, Halil Pasic wrote:
> On Mon, 4 Oct 2021 09:11:04 -0400
> "Michael S. Tsirkin" <mst@xxxxxxxxxx> wrote:
> 
> > > >> static inline bool virtio_access_is_big_endian(VirtIODevice *vdev)
> > > >> {
> > > >> #if defined(LEGACY_VIRTIO_IS_BIENDIAN)
> > > >>     return virtio_is_big_endian(vdev);
> > > >> #elif defined(TARGET_WORDS_BIGENDIAN)
> > > >>     if (virtio_vdev_has_feature(vdev, VIRTIO_F_VERSION_1)) {
> > > >>         /* Devices conforming to VIRTIO 1.0 or later are always LE. */
> > > >>         return false;
> > > >>     }
> > > >>     return true;
> > > >> #else
> > > >>     return false;
> > > >> #endif
> > > >> }
> > > >>   
> > > >
> > > > ok so that's a QEMU bug. Any virtio 1.0 and up
> > > > compatible device must use LE.
> > > > It can also present a legacy config space where the
> > > > endian depends on the guest.  
> > > 
> > > So, how is the virtio core supposed to determine this? A
> > > transport-specific callback?  
> > 
> > I'd say a field in VirtIODevice is easiest.
> 
> Wouldn't a call from transport code into virtio core
> be more handy? What I have in mind is stuff like vhost-user and vdpa. My
> understanding is, that for vhost setups where the config is outside qemu,
> we probably need a new  command that tells the vhost backend what
> endiannes to use for config. I don't think we can use
> VHOST_USER_SET_VRING_ENDIAN because  that one is on a virtqueue basis
> according to the doc. So for vhost-user and similar we would fire that
> command and probably also set the filed, while for devices for which
> control plane is handled by QEMU we would just set the field.
> 
> Does that sound about right?

I'm fine either way, but when would you invoke this?
With my idea backends can check the field when get_config
is invoked.

As for using this in VHOST, can we maybe re-use SET_FEATURES?

Kind of hacky but nice in that it will actually make existing backends
work...

-- 
MST