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

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Message-ID: <f6ee3ad3-0e75-2bc0-d03d-c2456ed6620a@linux.intel.com>
Date:   Sat, 5 Nov 2022 10:09:44 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>
Cc:     baolu.lu@...ux.intel.com, 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

On 2022/11/4 10:18, Tian, Kevin wrote:
>> 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.

It's not "DMA is blocked", but "DMA without PASID is blocked". :-)

As above term implies, it's conceptually incorrect to disable ATS/PRI
only because DMA without PASID is about to blocked.

> 
> 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.

Good catch!

How about moving iommu_enable/disable_pci_caps() into
iommu_probe/release_device() path? I may look into details if there's no
significant arch gaps.

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

Yes. Will update the commit message.

Best regards,
baolu