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

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Date: Mon, 8 Apr 2024 11:05:35 +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 4/8/24 11:03 AM, Tian, Kevin wrote:
>> From: Baolu Lu <baolu.lu@...ux.intel.com>
>> Sent: Sunday, April 7, 2024 3:28 PM
>>
>> 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.
>>
> 
> yes it's redundant. But what does "devices belonging to different
> IOMMU domains" in the original context try to convey? From above
> explanation it sounds irrelevant...

My typo. :-) Sorry for the confusion.

I should say,

"... attached to devices belonging to a same IOMMU ..."

Best regards,
baolu