linux-kernel - Re: [PATCH 00/12] Consolidate domain cache invalidation

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Date: Sun, 7 Apr 2024 15:28:19 +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, "Zhang, Tina" <tina.zhang@...el.com>,
 "Liu, Yi L" <yi.l.liu@...el.com>,
 "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
 "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 00/12] Consolidate domain cache invalidation

On 3/28/24 3:59 PM, Tian, Kevin wrote:
>> From: Lu Baolu<baolu.lu@...ux.intel.com>
>> Sent: Monday, March 25, 2024 10:17 AM
>>
>> The IOMMU hardware cache needs to be invalidated whenever the
>> mappings
>> in the domain are changed. Currently, domain cache invalidation is
>> scattered across different places, causing several issues:
>>
>> - IOMMU IOTLB Invalidation: This is done by iterating through the domain
>>    IDs of each domain using the following code:
>>
>>          xa_for_each(&dmar_domain->iommu_array, i, info)
>>                  iommu_flush_iotlb_psi(info->iommu, dmar_domain,
>>                                        start_pfn, nrpages,
>>                                        list_empty(&gather->freelist), 0);
>>
>>    This code could theoretically cause a use-after-free problem because
>>    there's no lock to protect the "info" pointer within the loop.
>>
>> - Inconsistent Invalidation Methods: Different domain types implement
>>    their own cache invalidation methods, making the code difficult to
>>    maintain. For example, the DMA domain, SVA domain, and nested domain
>>    have similar cache invalidation code scattered across different files.
>>
>> - SVA Domain Inconsistency: The SVA domain implementation uses a
>>    completely different data structure to track attached devices compared
>>    to other domains. This creates unnecessary differences and, even
>>    worse, leads to duplicate IOTLB invalidation when an SVA domain is
>>    attached to devices belonging to different IOMMU domains.
> can you elaborate how duplicated invalidations are caused?

Yes, sure.

Current Intel SVA implementation keeps the bond between mm and a PASID
of a device in a list of intel_svm_dev. In the mm notifier callback, it
iterates all intel_svam_dev in the list and invalidates the IOTLB and
device TLB sequentially.

If multiple devices belong to a single IOMMU, the IOTLB will be flushed
multiple times. However, since these devices share the same domain ID
and PASID, a single IOTLB cache invalidation is sufficient. The
additional flushes are redundant and negatively impact performance.

Best regards,
baolu