linux-kernel - Re: [PATCH v3 4/6] virtio_ccw: rev 1 devices set VIRTIO_F_VERSION

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Date:	Tue, 9 Dec 2014 18:23:19 +0100
From:	Cornelia Huck <cornelia.huck@...ibm.com>
To:	"Michael S. Tsirkin" <mst@...hat.com>
Cc:	linux-kernel@...r.kernel.org, dahi@...ux.vnet.ibm.com,
	rusty@...tcorp.com.au,
	Christian Borntraeger <borntraeger@...ibm.com>,
	linux390@...ibm.com, Martin Schwidefsky <schwidefsky@...ibm.com>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	linux-s390@...r.kernel.org
Subject: Re: [PATCH v3 4/6] virtio_ccw: rev 1 devices set VIRTIO_F_VERSION_1

On Tue, 9 Dec 2014 14:21:18 +0200
"Michael S. Tsirkin" <mst@...hat.com> wrote:

> On Tue, Dec 09, 2014 at 12:01:23PM +0100, Cornelia Huck wrote:
> > On Mon, 8 Dec 2014 15:06:03 +0200
> > "Michael S. Tsirkin" <mst@...hat.com> wrote:

> > > diff --git a/drivers/s390/kvm/virtio_ccw.c b/drivers/s390/kvm/virtio_ccw.c
> > > index 789275f..f9f87ba 100644
> > > --- a/drivers/s390/kvm/virtio_ccw.c
> > > +++ b/drivers/s390/kvm/virtio_ccw.c
> > > @@ -758,6 +758,13 @@ static int virtio_ccw_finalize_features(struct virtio_device *vdev)
> > >  	struct virtio_feature_desc *features;
> > >  	struct ccw1 *ccw;

This needs

+	struct virtio_device *vdev = &vcdev->vdev;

to make it compile :)

> > > 
> > > +	if (vcdev->revision == 1 &&
> > 
> > If we decide to keep this check, it should be for rev >= 1, though.
> 
> Fine, though this is theoretical, right?
> Ican change this with a patch on top.
> 
> > > +	    !__virtio_test_bit(vdev, VIRTIO_F_VERSION_1)) {
> > > +		dev_err(&vdev->dev, "virtio: device uses revision 1 "
> > > +			"but does not have VIRTIO_F_VERSION_1\n");
> > > +		return -EINVAL;
> > > +	}
> > > +
> > >  	ccw = kzalloc(sizeof(*ccw), GFP_DMA | GFP_KERNEL);
> > >  	if (!ccw)
> > >  		return 0;
> > 
> > I'm still not convinced by this change: I'd prefer to allow rev 1
> > without VERSION_1, especially as the core makes all its decisions based
> > upon VERSION_1.
> 
> At the moment, but this is an implementation detail.
> This is exactly why I want this hard requirement in code.
> 
> 
> > Unless someone else has a good argument in favour of
> > this change.
> 
> 
> Let's not commit to something we are not sure we
> can support.
> 
> We can always remove this code, but once we release
> guest we won't be able to drop it.
> 

OK, with your qemu patch on the host side this seems to be fine. No
further objections from me for now.

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