Re: [PATCH] iommu/virtio: Advertise IOMMU_CAP_CACHE_COHERENCY

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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@xxxxxxxxxx>
---
  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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