Re: [PATCH] iommu/virtio: Advertise IOMMU_CAP_CACHE_COHERENCY

[Jean-Philippe Brucker <jean-philippe@linaro.org>]

On Thu, Jul 14, 2022 at 01:01:37PM +0100, Robin Murphy wrote:
> On 2022-07-14 12:11, Jean-Philippe Brucker wrote:
> > Fix virtio-iommu interaction with VFIO, as VFIO now requires
> > IOMMU_CAP_CACHE_COHERENCY. virtio-iommu does not support non-cacheable
> > mappings, and always expects to be called with IOMMU_CACHE.
> 
> Can we know this is actually true though? What if the virtio-iommu
> implementation is backed by something other than VFIO, and the underlying
> hardware isn't coherent? AFAICS the spec doesn't disallow that.

Right, I should add a note about that. If someone does actually want to
support non-coherent device, I assume we'll add a per-device property, a
'non-cacheable' mapping flag, and IOMMU_CAP_CACHE_COHERENCY will hold.
I'm also planning to add a check on (IOMMU_CACHE && !IOMMU_NOEXEC) in
viommu_map(), but not as a fix.

In the meantime we do need to restore VFIO support under virtio-iommu,
since userspace still expects that to work, and the existing use-cases are
coherent devices.

Thanks,
Jean

> 
> Thanks,
> Robin.
> 
> > Fixes: e8ae0e140c05 ("vfio: Require that devices support DMA cache coherence")
> > Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
> > ---
> >   drivers/iommu/virtio-iommu.c | 11 +++++++++++
> >   1 file changed, 11 insertions(+)
> > 
> > diff --git a/drivers/iommu/virtio-iommu.c b/drivers/iommu/virtio-iommu.c
> > index 25be4b822aa0..bf340d779c10 100644
> > --- a/drivers/iommu/virtio-iommu.c
> > +++ b/drivers/iommu/virtio-iommu.c
> > @@ -1006,7 +1006,18 @@ static int viommu_of_xlate(struct device *dev, struct of_phandle_args *args)
> >   	return iommu_fwspec_add_ids(dev, args->args, 1);
> >   }
> > +static bool viommu_capable(enum iommu_cap cap)
> > +{
> > +	switch (cap) {
> > +	case IOMMU_CAP_CACHE_COHERENCY:
> > +		return true;
> > +	default:
> > +		return false;
> > +	}
> > +}
> > +
> >   static struct iommu_ops viommu_ops = {
> > +	.capable		= viommu_capable,
> >   	.domain_alloc		= viommu_domain_alloc,
> >   	.probe_device		= viommu_probe_device,
> >   	.probe_finalize		= viommu_probe_finalize,
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