linux-kernel - Re: [PATCH 0/6] dma mapping/iommu: Allow IOMMU IOVA rcache range to be configured

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Message-ID: <20210319134047.GA5729@lst.de>
Date:   Fri, 19 Mar 2021 14:40:47 +0100
From:   Christoph Hellwig <hch@....de>
To:     John Garry <john.garry@...wei.com>
Cc:     joro@...tes.org, will@...nel.org, jejb@...ux.ibm.com,
        martin.petersen@...cle.com, hch@....de, m.szyprowski@...sung.com,
        robin.murphy@....com, iommu@...ts.linux-foundation.org,
        linux-kernel@...r.kernel.org, linux-scsi@...r.kernel.org,
        linuxarm@...wei.com
Subject: Re: [PATCH 0/6] dma mapping/iommu: Allow IOMMU IOVA rcache range
 to be configured

On Fri, Mar 19, 2021 at 09:25:42PM +0800, John Garry wrote:
> For streaming DMA mappings involving an IOMMU and whose IOVA len regularly
> exceeds the IOVA rcache upper limit (meaning that they are not cached),
> performance can be reduced. 
> 
> This is much more pronounced from commit 4e89dce72521 ("iommu/iova: Retry
> from last rb tree node if iova search fails"), as discussed at [0].
> 
> IOVAs which cannot be cached are highly involved in the IOVA aging issue,
> as discussed at [1].

I'm confused.  If this a limit in the IOVA allocator, dma-iommu should
be able to just not grow the allocation so larger without help from
the driver.

If contrary to the above description it is device-specific, the driver
could simply use dma_get_max_seg_size().