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    SubjectRe: [PATCH v6 02/12] iommu: Add pasid_bits field in struct dev_iommu
    On Tue, May 10, 2022 at 02:17:28PM +0800, Lu Baolu wrote:

    > int iommu_device_register(struct iommu_device *iommu,
    > diff --git a/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
    > index 627a3ed5ee8f..afc63fce6107 100644
    > +++ b/drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c
    > @@ -2681,6 +2681,8 @@ static struct iommu_device *arm_smmu_probe_device(struct device *dev)
    > smmu->features & ARM_SMMU_FEAT_STALL_FORCE)
    > master->stall_enabled = true;
    >
    > + dev->iommu->pasid_bits = master->ssid_bits;
    > return &smmu->iommu;
    >
    > err_free_master:
    > diff --git a/drivers/iommu/intel/iommu.c b/drivers/iommu/intel/iommu.c
    > index 2990f80c5e08..99643f897f26 100644
    > +++ b/drivers/iommu/intel/iommu.c
    > @@ -4624,8 +4624,11 @@ static struct iommu_device *intel_iommu_probe_device(struct device *dev)
    > if (pasid_supported(iommu)) {
    > int features = pci_pasid_features(pdev);
    >
    > - if (features >= 0)
    > + if (features >= 0) {
    > info->pasid_supported = features | 1;
    > + dev->iommu->pasid_bits =
    > + fls(pci_max_pasids(pdev)) - 1;
    > + }

    It is not very nice that both the iommu drivers have to duplicate the
    code to read the pasid capability out of the PCI device.

    IMHO it would make more sense for the iommu layer to report the
    capability of its own HW block only, and for the core code to figure
    out the master's limitation using a bus-specific approach.

    It is also unfortunate that the enable/disable pasid is inside the
    iommu driver as well - ideally the PCI driver itself would do this
    when it knows it wants to use PASIDs.

    The ordering interaction with ATS makes this look quite annoying
    though. :(

    I'm also not convinced individual IOMMU drivers should be forcing ATS
    on, there are performance and functional implications here. Using ATS
    or not is possibly best left as an administrator policy controlled by
    the core code. Again we seem to have some mess.

    Jason

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