Re: [PATCH v4 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group

[Robin Murphy]

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 <jgg@xxxxxxxxxx>
> Reviewed-by: Kevin Tian <kevin.tian@xxxxxxxxx>
> Signed-off-by: Nicolin Chen <nicolinc@xxxxxxxxxx>
> ---
[...]
> diff --git a/drivers/iommu/arm/arm-smmu/arm-smmu.c b/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!)

Thanks,
Robin.

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