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

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Date:   Tue, 5 Oct 2021 09:25:39 +0200
From:   Halil Pasic <pasic@...ux.ibm.com>
To:     "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Cornelia Huck <cohuck@...hat.com>,
        Jason Wang <jasowang@...hat.com>,
        Xie Yongji <xieyongji@...edance.com>,
        virtualization@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, markver@...ibm.com,
        Christian Borntraeger <borntraeger@...ibm.com>,
        linux-s390@...r.kernel.org, qemu-devel@...gnu.org,
        Halil Pasic <pasic@...ux.ibm.com>
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify

On Mon, 4 Oct 2021 09:11:04 -0400
"Michael S. Tsirkin" <mst@...hat.com> 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?