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Date:   Mon, 5 Jun 2023 09:54:57 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     Shannon Nelson <shannon.nelson@amd.com>,
        virtualization@lists.linux-foundation.org, netdev@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>,
        Eugenio =?iso-8859-1?Q?P=E9rez?= <eperezma@redhat.com>,
        Tiwei Bie <tiwei.bie@intel.com>, kvm@vger.kernel.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] vhost-vdpa: filter VIRTIO_F_RING_PACKED feature
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On Mon, Jun 05, 2023 at 03:30:35PM +0200, Stefano Garzarella wrote:
> On Mon, Jun 05, 2023 at 09:00:25AM -0400, Michael S. Tsirkin wrote:
> > On Mon, Jun 05, 2023 at 02:54:20PM +0200, Stefano Garzarella wrote:
> > > On Mon, Jun 05, 2023 at 08:41:54AM -0400, Michael S. Tsirkin wrote:
> > > > On Mon, Jun 05, 2023 at 01:06:44PM +0200, Stefano Garzarella wrote:
> > > > > vhost-vdpa IOCTLs (eg. VHOST_GET_VRING_BASE, VHOST_SET_VRING_BASE)
> > > > > don't support packed virtqueue well yet, so let's filter the
> > > > > VIRTIO_F_RING_PACKED feature for now in vhost_vdpa_get_features().
> > > > >
> > > > > This way, even if the device supports it, we don't risk it being
> > > > > negotiated, then the VMM is unable to set the vring state properly.
> > > > >
> > > > > Fixes: 4c8cf31885f6 ("vhost: introduce vDPA-based backend")
> > > > > Cc: stable@vger.kernel.org
> > > > > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> > > > > ---
> > > > >
> > > > > Notes:
> > > > >     This patch should be applied before the "[PATCH v2 0/3] vhost_vdpa:
> > > > >     better PACKED support" series [1] and backported in stable branches.
> > > > >
> > > > >     We can revert it when we are sure that everything is working with
> > > > >     packed virtqueues.
> > > > >
> > > > >     Thanks,
> > > > >     Stefano
> > > > >
> > > > >     [1] https://lore.kernel.org/virtualization/20230424225031.18947-1-shannon.nelson@amd.com/
> > > >
> > > > I'm a bit lost here. So why am I merging "better PACKED support" then?
> > > 
> > > To really support packed virtqueue with vhost-vdpa, at that point we would
> > > also have to revert this patch.
> > > 
> > > I wasn't sure if you wanted to queue the series for this merge window.
> > > In that case do you think it is better to send this patch only for stable
> > > branches?
> > > > Does this patch make them a NOP?
> > > 
> > > Yep, after applying the "better PACKED support" series and being
> > > sure that
> > > the IOCTLs of vhost-vdpa support packed virtqueue, we should revert this
> > > patch.
> > > 
> > > Let me know if you prefer a different approach.
> > > 
> > > I'm concerned that QEMU uses vhost-vdpa IOCTLs thinking that the kernel
> > > interprets them the right way, when it does not.
> > > 
> > > Thanks,
> > > Stefano
> > > 
> > 
> > If this fixes a bug can you add Fixes tags to each of them? Then it's ok
> > to merge in this window. Probably easier than the elaborate
> > mask/unmask dance.
> 
> CCing Shannon (the original author of the "better PACKED support"
> series).
> 
> IIUC Shannon is going to send a v3 of that series to fix the
> documentation, so Shannon can you also add the Fixes tags?
> 
> Thanks,
> Stefano

Well this is in my tree already. Just reply with
Fixes: <>
to each and I will add these tags.

If I start dropping and rebasing this won't make it in this window.

-- 
MST