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Date: Mon, 15 Jun 2020 12:37:25 +0200
From: Halil Pasic <pasic@linux.ibm.com>
Subject: Re: [PATCH] s390: protvirt: virtio: Refuse device without IOMMU
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To: Jason Wang <jasowang@redhat.com>
Cc: Pierre Morel <pmorel@linux.ibm.com>, linux-kernel@vger.kernel.org, borntraeger@de.ibm.com, frankja@linux.ibm.com, mst@redhat.com, cohuck@redhat.com, kvm@vger.kernel.org, linux-s390@vger.kernel.org, virtualization@lists.linux-foundation.org

On Mon, 15 Jun 2020 11:01:55 +0800
Jason Wang <jasowang@redhat.com> wrote:

> > hum, in between I found another way which seems to me much better:
> >
> > We already have the force_dma_unencrypted() function available which 
> > AFAIU is what we want for encrypted memory protection and is already 
> > used by power and x86 SEV/SME in a way that seems AFAIU compatible 
> > with our problem.
> >
> > Even DMA and IOMMU are different things, I think they should be used 
> > together in our case.
> >
> > What do you think?
> >
> > The patch would then be something like:
> >
> > diff --git a/drivers/virtio/virtio.c b/drivers/virtio/virtio.c
> > index a977e32a88f2..53476d5bbe35 100644
> > --- a/drivers/virtio/virtio.c
> > +++ b/drivers/virtio/virtio.c
> > @@ -4,6 +4,7 @@
> >  #include <linux/virtio_config.h>
> >  #include <linux/module.h>
> >  #include <linux/idr.h>
> > +#include <linux/dma-direct.h>
> >  #include <uapi/linux/virtio_ids.h>
> >
> >  /* Unique numbering for virtio devices. */
> > @@ -179,6 +180,10 @@ int virtio_finalize_features(struct virtio_device 
> > *dev)
> >         if (!virtio_has_feature(dev, VIRTIO_F_VERSION_1))
> >                 return 0;
> >
> > +       if (force_dma_unencrypted(&dev->dev) &&
> > +           !virtio_has_feature(dev, VIRTIO_F_IOMMU_PLATFORM))
> > +               return -EIO;
> > +
> >         virtio_add_status(dev, VIRTIO_CONFIG_S_FEATURES_OK);
> >         status = dev->config->get_status(dev);
> >         if (!(status & VIRTIO_CONFIG_S_FEATURES_OK)) {  
> 
> 
> I think this can work but need to listen from Michael

I don't think Christoph Hellwig will like force_dma_unencrypted()
in virtio code:
https://lkml.org/lkml/2020/2/20/630

Regards,
Halil