linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Make context clearing consistent with context mapping

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Date:   Tue, 14 Nov 2023 11:22:19 +0800
From:   Baolu Lu <baolu.lu@...ux.intel.com>
To:     "Tian, Kevin" <kevin.tian@...el.com>,
        Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
        Robin Murphy <robin.murphy@....com>,
        Jason Gunthorpe <jgg@...pe.ca>
Cc:     baolu.lu@...ux.intel.com,
        "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] iommu/vt-d: Make context clearing consistent with
 context mapping

On 11/14/23 11:20 AM, Tian, Kevin wrote:
>> From: Lu Baolu <baolu.lu@...ux.intel.com>
>> Sent: Tuesday, November 14, 2023 9:11 AM
>>
>> In the iommu probe_device path, domain_context_mapping() allows setting
>> up the context entry for a non-PCI device. However, in the iommu
>> release_device path, domain_context_clear() only clears context entries
>> for PCI devices.
>>
>> Make domain_context_clear() behave consistently with
>> domain_context_mapping() by clearing context entries for both PCI and
>> non-PCI devices.
>>
>> Fixes: 579305f75d34 ("iommu/vt-d: Update to use PCI DMA aliases")
>> Signed-off-by: Lu Baolu <baolu.lu@...ux.intel.com>
> 
> The code before the fix tag also has the same problem. If we really want
> backport then let's find out the very first commit which exhibits this
> problem.

Commit 579305f75d34 allows non-PCI devices.

+       if (!dev_is_pci(dev))
+               return domain_context_mapping_one(domain, iommu, bus, devfn,
                                                   translation);
+
+       data.domain = domain;
+       data.iommu = iommu;
+       data.translation = translation;
+
+       return pci_for_each_dma_alias(to_pci_dev(dev),
+                                     &domain_context_mapping_cb, &data);

But it forgot to update the domain_context_clear() helper. So this is
actually a fix for that commit.

> 
> But I wonder the actual impact w/o such fix. If there is no hot-remove
> possible for those non-PCI devices the context entry will be leaved
> enabled until the machine is off. Then this fix is nice-to-have then
> probably no need to backport?

It doesn't cause real issues as far as I can see. So there's no need to
back port it to stable kernels. That's the reason I didn't add cc-stable
tag. But we still need a fix tag as it's actually a fix.

Best regards,
baolu