On 2022-06-30 21:36, Nicolin Chen wrote:
Cases like VFIO wish to attach a device to an existing domain that was
not allocated specifically from the device. This raises a condition
where the IOMMU driver can fail the domain attach because the domain and
device are incompatible with each other.

This is a soft failure that can be resolved by using a different domain.

Provide a dedicated errno from the IOMMU driver during attach that the
reason attached failed is because of domain incompatability. EMEDIUMTYPE
is chosen because it is never used within the iommu subsystem today and
evokes a sense that the 'medium' aka the domain is incompatible.

VFIO can use this to know attach is a soft failure and it should continue
searching. Otherwise the attach will be a hard failure and VFIO will
return the code to userspace.

Update all drivers to return EMEDIUMTYPE in their failure paths that are
related to domain incompatability. Also remove adjacent error prints for
these soft failures, to prevent a kernel log spam, since -EMEDIUMTYPE is
clear enough to indicate an incompatability error.

Add kdocs describing this behavior.

Suggested-by: Jason Gunthorpe <j...@nvidia.com>
Reviewed-by: Kevin Tian <kevin.t...@intel.com>
Signed-off-by: Nicolin Chen <nicol...@nvidia.com>
diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c 
index 2ed3594f384e..072cac5ab5a4 100644
--- a/drivers/iommu/arm/arm-smmu/arm-smmu.c
+++ b/drivers/iommu/arm/arm-smmu/arm-smmu.c
@@ -1135,10 +1135,8 @@ static int arm_smmu_attach_dev(struct iommu_domain 
*domain, struct device *dev)
        struct arm_smmu_device *smmu;
        int ret;
- if (!fwspec || fwspec->ops != &arm_smmu_ops) {
-               dev_err(dev, "cannot attach to SMMU, is it on the same bus?\n");
-               return -ENXIO;
-       }
+       if (!fwspec || fwspec->ops != &arm_smmu_ops)
+               return -EMEDIUMTYPE;

This is the wrong check, you want the "if (smmu_domain->smmu != smmu)" condition further down. If this one fails it's effectively because the device doesn't have an IOMMU at all, and similar to patch #3 it will be removed once the core code takes over properly (I even have both those patches written now!)


         * FIXME: The arch/arm DMA API code tries to attach devices to its own
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