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

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Date:   Thu, 3 Jun 2021 08:39:29 +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 6/2/21 3:48 PM, John Garry wrote:
> On 02/06/2021 05:37, Lu Baolu wrote:
>> 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.
> 
> Hi Baolu,
> 
> I did actually try increasing the range for a 'live' domain in the v1 
> series, but it turned out too messy. First problem is reallocating the 
> memory to hold the rcaches. Second problem is that we need to deal with 
> the issue that all IOVAs in the rcache need to be a pow-of-2, which is 
> difficult to enforce for IOVAs which weren't being cached before, but 
> now would be.
> 
> So now I changed to work similar to how we change the default domain 
> type, i.e. don't operate on a 'live' domain.

If these hard restrictions on users are just to walk around the messy
code in kernel, I would rather solve those messy code to achieve a
better user experience. :-)

Best regards,
baolu