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

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Message-ID: <08c9f2a2-b2fc-2574-e10a-b6dbd0caedb0@arm.com>
Date:   Thu, 13 Apr 2023 11:44:56 +0100
From:   Robin Murphy <robin.murphy@....com>
To:     John Garry <john.g.garry@...cle.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 2023-04-13 11:16, John Garry wrote:
> 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.

Oh, thanks for the reminder - IIRC this wants a minor rebase now, let me 
double-check and send a v4 that applies cleanly...

Cheers,
Robin.