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

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Date:   Wed, 14 Jul 2021 17:24:29 +0300
From:   Georgi Djakov <djakov@...nel.org>
To:     Georgi Djakov <quic_c_gdjako@...cinc.com>, will@...nel.org,
        robin.murphy@....com
Cc:     joro@...tes.org, isaacm@...eaurora.org, baolu.lu@...ux.intel.com,
        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 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.

Thanks,
Georgi