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

From: "Tian, Kevin" <kevin.tian@intel.com>
To: Nicolin Chen <nicolinc@nvidia.com>,
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Subject: RE: [PATCH 1/5] iommu: Return -EMEDIUMTYPE for incompatible domain and device/group
Date: Wed, 8 Jun 2022 07:49:10 +0000	[thread overview]
Message-ID: <BN9PR11MB52763602BBEEEF580C3B360E8CA49@BN9PR11MB5276.namprd11.prod.outlook.com> (raw)
In-Reply-To: <20220606061927.26049-2-nicolinc@nvidia.com>

> From: Nicolin Chen
> Sent: Monday, June 6, 2022 2:19 PM
> 
> 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.

Seems not all drivers are converted, e.g.:

mtk_iommu_v1_attach_device():
	/* Only allow the domain created internally. */
	mtk_mapping = data->mapping;
	if (mtk_mapping->domain != domain)
		return 0;
** the current code sounds incorrect which should return an error

s390_iommu_attach_device():
	/* Allow only devices with identical DMA range limits */
	} else if (domain->geometry.aperture_start != zdev->start_dma ||
		domain->geometry.aperture_end != zdev->end_dma) {
		rc = -EINVAL;

sprd_iommu_attach_device():
	if (dom->sdev) {
		pr_err("There's already a device attached to this domain.\n");
		return -EINVAL;
	}

gart_iommu_attach_dev():
	if (gart->active_domain && gart->active_domain != domain) {
		ret = -EBUSY;

arm_smmu_attach_dev():
	if (!fwspec || fwspec->ops != &arm_smmu_ops) {
		dev_err(dev, "cannot attach to SMMU, is it on the same bus?\n");
		return -ENXIO;
	}
**probably this check can be covered by next patch which moves bus ops
check into iommu core?

Thanks
Kevin