linux-kernel - Re: [PATCH v3 0/7] iommu: Allow IOVA rcache range be configured

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Message-ID: <834ad35a-7310-1738-7d17-7c061ca73e4c@linux.intel.com>
Date:   Wed, 2 Jun 2021 12:37:20 +0800
From:   Lu Baolu <baolu.lu@...ux.intel.com>
To:     John Garry <john.garry@...wei.com>, joro@...tes.org,
        will@...nel.org, robin.murphy@....com
Cc:     baolu.lu@...ux.intel.com, iommu@...ts.linux-foundation.org,
        linuxarm@...wei.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 0/7] iommu: Allow IOVA rcache range be configured

On 6/1/21 10:29 PM, 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 ageing issue,
> as discussed at [1].
> 
> This series allows the IOVA rcache range be configured, so that we may
> cache all IOVAs per domain, thus improving performance.
> 
> A new IOMMU group sysfs file is added - max_opt_dma_size - which is used
> indirectly to configure the IOVA rcache range:
> /sys/kernel/iommu_groups/X/max_opt_dma_size
> 
> This file is updated same as how the IOMMU group default domain type is
> updated, i.e. must unbind the only device in the group first.

Could you explain why it requires singleton group and driver unbinding
if the user only wants to increase the upper limit? I haven't dived into
the details yet, sorry if this is a silly question.

Best regards,
baolu