linux-kernel - RE: [PATCH 1/1] iommu/vt-d: Enable PASID during iommu device probe

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Date:   Tue, 13 Sep 2022 03:13:58 +0000
From:   "Tian, Kevin" <kevin.tian@...el.com>
To:     Lu Baolu <baolu.lu@...ux.intel.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
CC:     Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        "Robin Murphy" <robin.murphy@....com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/1] iommu/vt-d: Enable PASID during iommu device probe

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Monday, September 12, 2022 10:48 AM
> 
> Previously PASID supports on both IOMMU and PCI devices are enabled in
> the
> iommu_dev_enable_feature(dev, IOMMU_DEV_FEAT_SVA) path. It's
> functionally
> correct as the SVA is the only feature that requires PASID setup. However,
> looking ahead, we will add more features that need to enable pasid (for
> example, kernel DMA with PASID, SIOV, VM guest SVA, etc.). It makes more
> sense to enable PASID during iommu probing device.
> 
> This enables PASID during iommu probing device and deprecates the
> intel_iommu_enable_pasid() helper. This is safe because the IOMMU
> hardware
> will block any PCI TLP with a PASID prefix if there is no IOMMU domain
> attached to the PASID of the device.
> 

IMHO it's better to enable something only when it's actually required,
e.g. does it make more sense to have a IOMMU_DEV_FEAT_PASID
instead?

What this patch does has one problem. It's an intel-iommu driver
internal policy. How can a device driver reliably tell that the pasid
capability has been enabled by the iommu driver?

Thanks
Kevin