linux-kernel - RE: [PATCH 3/7] iommu/vt-d: Use device_block_translation() in dev

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Message-ID: <BN9PR11MB52763EB6CDF2E171210D04E48C3B9@BN9PR11MB5276.namprd11.prod.outlook.com>
Date:   Fri, 4 Nov 2022 02:18:44 +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>,
        "Liu, Yi L" <yi.l.liu@...el.com>,
        "Pan, Jacob jun" <jacob.jun.pan@...el.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 3/7] iommu/vt-d: Use device_block_translation() in
 dev_attach error path

> From: Lu Baolu <baolu.lu@...ux.intel.com>
> Sent: Thursday, November 3, 2022 1:53 PM
> 
> If domain attaching to device fails, the IOMMU driver should bring the
> device to blocking DMA state. The upper layer is expected to recover it
> by attaching a new domain. Use device_block_translation() in the error
> path of dev_attach to make the behavior specific.
> 
> The difference between device_block_translation() and the previous
> dmar_remove_one_dev_info() is that the latter disables PCIe ATS and the
> related PCIe features. This is unnecessary as these features are not per
> domain capabilities, disabling them during domain switching is
> unnecessary.

well, the opposite argument is that when the DMA is blocked what is
the point of enabling ATS/PRI on the device.

btw this change is partial. @attach_dev still calls iommu_enable_pci_caps()
which always tries to enable PCI capabilities w/o checking whether they
have been enabled or not. Then user will hit -EBUSY when related PCI
helpers are called.

another difference worthy of pointing out is that in scalable mode it is
the RID2PASID entry instead of context entry being cleared.