linux-kernel - Re: [PATCH v7 00/15] Optimizing iommu

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Date:   Thu, 15 Jul 2021 09:23:22 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     Georgi Djakov <djakov@...nel.org>,
        Georgi Djakov <quic_c_gdjako@...cinc.com>, will@...nel.org,
        robin.murphy@....com
Cc:     baolu.lu@...ux.intel.com, joro@...tes.org, isaacm@...eaurora.org,
        pratikp@...eaurora.org, iommu@...ts.linux-foundation.org,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7 00/15] Optimizing iommu_[map/unmap] performance

On 7/14/21 10:24 PM, Georgi Djakov wrote:
> On 16.06.21 16:38, Georgi Djakov wrote:
>> When unmapping a buffer from an IOMMU domain, the IOMMU framework unmaps
>> the buffer at a granule of the largest page size that is supported by
>> the IOMMU hardware and fits within the buffer. For every block that
>> is unmapped, the IOMMU framework will call into the IOMMU driver, and
>> then the io-pgtable framework to walk the page tables to find the entry
>> that corresponds to the IOVA, and then unmaps the entry.
>>
>> This can be suboptimal in scenarios where a buffer or a piece of a
>> buffer can be split into several contiguous page blocks of the same size.
>> For example, consider an IOMMU that supports 4 KB page blocks, 2 MB page
>> blocks, and 1 GB page blocks, and a buffer that is 4 MB in size is being
>> unmapped at IOVA 0. The current call-flow will result in 4 indirect 
>> calls,
>> and 2 page table walks, to unmap 2 entries that are next to each other in
>> the page-tables, when both entries could have been unmapped in one shot
>> by clearing both page table entries in the same call.
>>
>> The same optimization is applicable to mapping buffers as well, so
>> these patches implement a set of callbacks called unmap_pages and
>> map_pages to the io-pgtable code and IOMMU drivers which unmaps or maps
>> an IOVA range that consists of a number of pages of the same
>> page size that is supported by the IOMMU hardware, and allows for
>> manipulating multiple page table entries in the same set of indirect
>> calls. The reason for introducing these callbacks is to give other IOMMU
>> drivers/io-pgtable formats time to change to using the new callbacks, so
>> that the transition to using this approach can be done piecemeal.
> 
> Hi Will,
> 
> Did you get a chance to look at this patchset? Most patches are already
> acked/reviewed and all still applies clean on rc1.

I also have the ops->[un]map_pages implementation for the Intel IOMMU
driver. I will post them once the iommu/core part get applied.

Best regards,
baolu