Re: [PATCH] iommu/virtio: Advertise IOMMU_CAP_CACHE_COHERENCY

[Robin Murphy <robin.murphy@arm.com>]

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@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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