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Date:   Tue, 5 Oct 2021 07:11:32 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Halil Pasic <pasic@linux.ibm.com>
Cc:     linux-s390@vger.kernel.org, markver@us.ibm.com,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        qemu-devel@nongnu.org, Jason Wang <jasowang@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        linux-kernel@vger.kernel.org,
        virtualization@lists.linux-foundation.org,
        Xie Yongji <xieyongji@bytedance.com>
Subject: Re: [RFC PATCH 1/1] virtio: write back features before verify
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On Tue, Oct 05, 2021 at 12:46:34PM +0200, Halil Pasic wrote:
> On Tue, 5 Oct 2021 03:53:17 -0400
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> 
> > > 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...
> 
> Basically the equivalent of this patch, just on the vhost interface,
> right? Could work I have to look into it :)

yep