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Date:   Mon, 29 Jun 2020 18:05:26 +0200
From:   Cornelia Huck <cohuck@redhat.com>
To:     "Michael S. Tsirkin" <mst@redhat.com>
Cc:     Pierre Morel <pmorel@linux.ibm.com>, linux-kernel@vger.kernel.org,
        pasic@linux.ibm.com, borntraeger@de.ibm.com, frankja@linux.ibm.com,
        jasowang@redhat.com, kvm@vger.kernel.org,
        linux-s390@vger.kernel.org,
        virtualization@lists.linux-foundation.org, thomas.lendacky@amd.com,
        david@gibson.dropbear.id.au, linuxram@us.ibm.com,
        heiko.carstens@de.ibm.com, gor@linux.ibm.com
Subject: Re: [PATCH v3 1/1] s390: virtio: let arch accept devices without
 IOMMU feature
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On Mon, 29 Jun 2020 11:57:14 -0400
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Wed, Jun 17, 2020 at 12:43:57PM +0200, Pierre Morel wrote:
> > An architecture protecting the guest memory against unauthorized host
> > access may want to enforce VIRTIO I/O device protection through the
> > use of VIRTIO_F_IOMMU_PLATFORM.
> > 
> > Let's give a chance to the architecture to accept or not devices
> > without VIRTIO_F_IOMMU_PLATFORM.
> > 
> > Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
> > Acked-by: Jason Wang <jasowang@redhat.com>
> > Acked-by: Christian Borntraeger <borntraeger@de.ibm.com>
> > ---
> >  arch/s390/mm/init.c     |  6 ++++++
> >  drivers/virtio/virtio.c | 22 ++++++++++++++++++++++
> >  include/linux/virtio.h  |  2 ++
> >  3 files changed, 30 insertions(+)

> > @@ -179,6 +194,13 @@ int virtio_finalize_features(struct virtio_device *dev)
> >  	if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
> >  		return 0;
> >  
> > +	if (arch_needs_virtio_iommu_platform(dev) &&
> > +		!virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM)) {
> > +		dev_warn(&dev->dev,
> > +			 "virtio: device must provide VIRTIO_F_IOMMU_PLATFORM\n");
> > +		return -ENODEV;
> > +	}
> > +
> >  	virtio_add_status(dev, VIRTIO_CONFIG_S_FEATURES_OK);
> >  	status = dev->config->get_status(dev);
> >  	if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {  
> 
> Well don't you need to check it *before* VIRTIO_F_VERSION_1, not after?

But it's only available with VERSION_1 anyway, isn't it? So it probably
also needs to fail when this feature is needed if VERSION_1 has not been
negotiated, I think.