Re: [PATCH RFC 05/12] remoteproc: always report VIRTIO_F_VERSION_1 feature

[Mathieu Poirier <mathieu.poirier@linaro.org>]

On Tue, Sep 22, 2026 at 08:32:15AM +0200, Francesco Valla wrote:
> On Mon, Sep 21, 2026 at 09:47:54AM -0600, Mathieu Poirier wrote:
> > On Wed, Sep 16, 2026 at 11:10:50PM +0200, Francesco Valla wrote:
> > > There is currently no way to report the VIRTIO_F_VERSION_1 using the
> > > resource table alone, as the per-vdev feature array is limited to 32
> > > bits.
> > > 
> > > Considering that the VirtIO 1.0 specification is now ~10 years old,
> > > always report the VIRTIO_F_VERSION_1 feature, as some drivers depend on
> > > it.
> > > 
> > > Signed-off-by: Francesco Valla <francesco@valla.it>
> > > ---
> > >  drivers/remoteproc/remoteproc_virtio.c | 10 ++++++----
> > >  1 file changed, 6 insertions(+), 4 deletions(-)
> > > 
> > > diff --git a/drivers/remoteproc/remoteproc_virtio.c b/drivers/remoteproc/remoteproc_virtio.c
> > > index 74e0da970f1d..cfd66d9d1c9e 100644
> > > --- a/drivers/remoteproc/remoteproc_virtio.c
> > > +++ b/drivers/remoteproc/remoteproc_virtio.c
> > > @@ -249,7 +249,7 @@ static u64 rproc_virtio_get_features(struct virtio_device *vdev)
> > >  
> > >  	rsc = (void *)rvdev->rproc->table_ptr + rvdev->rsc_offset;
> > >  
> > > -	return rsc->dfeatures;
> > > +	return rsc->dfeatures | (1ULL << VIRTIO_F_VERSION_1);
> > 
> > I understand this.
> > 
> > >  }
> > >  
> > >  static void rproc_transport_features(struct virtio_device *vdev)
> > > @@ -275,14 +275,16 @@ static int rproc_virtio_finalize_features(struct virtio_device *vdev)
> > >  	/* Give virtio_rproc a chance to accept features. */
> > >  	rproc_transport_features(vdev);
> > >  
> > > -	/* Make sure we don't have any features > 32 bits! */
> > > -	BUG_ON((u32)vdev->features != vdev->features);
> > > +	/* Make sure we don't have any features > 32 bits except VIRTIO_F_VERSION_1 */
> > > +	if (WARN_ON_ONCE((u32)vdev->features !=
> > > +			 (vdev->features & ~(1ULL << VIRTIO_F_VERSION_1))))
> > > +		return -1;
> > 
> > And this.
> > 
> > >  
> > >  	/*
> > >  	 * Remember the finalized features of our vdev, and provide it
> > >  	 * to the remote processor once it is powered on.
> > >  	 */
> > > -	rsc->gfeatures = vdev->features;
> > > +	rsc->gfeatures = vdev->features & ~(1ULL << VIRTIO_F_VERSION_1);
> > >  
> > 
> > But I'm having trouble with this one.  
> > 
> > First bit 32 of vdev->features gets cleared and then bit 0 to 31 are assigned to
> > rsc->gfeatures.  Why bother with clearing bit 32?
> >
> 
> Bits > 31 gets silently dropped because rsc->gfeatures is a u32; this
> was inserted to explicitly drop the flag that has been inserted before
> and avoid the "silently". It has no real funtional impact and can be
> safely dropped.
>

We have come to the same conclusion.
 
> > >  	return 0;
> > >  }
> > > 
> > > -- 
> > > 2.55.0
> > >