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

[Francesco Valla <francesco@valla.it>]

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.

> >  	return 0;
> >  }
> > 
> > -- 
> > 2.55.0
> >