RE: [PATCH v2 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group

[Tian, Kevin]

> From: Nicolin Chen <nicolinc@xxxxxxxxxx>
> Sent: Thursday, June 16, 2022 8:03 AM
> 
> 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 turn adjacent error prints into
> debug prints, for these soft failures, to prevent a kernel log spam.
> 
> Add kdocs describing this behavior.
> 
> Suggested-by: Jason Gunthorpe <jgg@xxxxxxxxxx>
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>

Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>