linux-kernel - Re: [PATCH v3] iommu: Optimise PCI SAC address trick

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Message-ID: <02eebcda-f60e-f8ed-7057-cf293d15a173@oracle.com>
Date:   Thu, 13 Apr 2023 11:16:22 +0100
From:   John Garry <john.g.garry@...cle.com>
To:     Robin Murphy <robin.murphy@....com>, joro@...tes.org
Cc:     will@...nel.org, iommu@...ts.linux.dev,
        linux-kernel@...r.kernel.org,
        Linus Torvalds <torvalds@...ux-foundation.org>, kuba@...nel.org
Subject: Re: [PATCH v3] iommu: Optimise PCI SAC address trick

On 20/01/2023 11:33, John Garry wrote:
> On 18/01/2023 17:26, Robin Murphy wrote:
>> Per the reasoning in commit 4bf7fda4dce2 ("iommu/dma: Add config for
>> PCI SAC address trick") and its subsequent revert, this mechanism no
>> longer serves its original purpose, but now only works around broken
>> hardware/drivers in a way that is unfortunately too impactful to remove.
>>
>> This does not, however prevent us from solving the performance impact
>> which the workaround imposes on large-scale systems that don't need it.
>> That is felt once the 32-bit IOVA space fills up and we keep
>> unsuccessfully trying to allocate from it. However, if we get to that
>> point then in fact it's already the endgame. The nature of the allocator
>> is such that the first IOVA we give to a device after the 32-bit space
>> runs out will be the highest possible address for that device, ever.
>> If that works, then great, we can be pretty sure it's safe to optimise
>> for speed by always allocating from the full range. And if it doesn't,
>> then the worst has already happened and any brokenness is now showing,
>> so there's no point continuing to try to hide it.
>>
>> To that end, implement a flag to refine this into a per-device policy
>> that can automatically get itself out of the way if and when it stops
>> being useful.
>>
>> CC: John Garry <john.garry@...wei.com>
>> CC: Linus Torvalds <torvalds@...ux-foundation.org>
>> Signed-off-by: Robin Murphy <robin.murphy@....com>
>> ---
>>
>> v3: Expand the flag name, add a print with inline commentary for good
>>      measure, and refactor the code flow even more (too many ifs and
>>      indents...) such that I didn't presume to carry forward John's R-b.
> 
> I like the new changes, so feel free to add:
> 
> Reviewed-by: John Garry <john.g.garry@...cle.com>

Is there any chance that this can be picked up?

I also saw that it fixed an issue for Jakub (cc'ed) recently.