Virtualization - Jul 2022 - [PATCH] iommu/virtio: Advertise IOMMU_CAP_CACHE_COHERENCY

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.

Thanks,
Robin.
> Fixes: e8ae0e140c05 ("vfio: Require that devices support DMA cache
coherence")
> Signed-off-by: Jean-Philippe Brucker <jean-philippe at 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,

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 at
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,