From: John Garry <john.g.garry@oracle.com>
To: Robin Murphy <robin.murphy@arm.com>, joro@8bytes.org
Cc: will@kernel.org, iommu@lists.linux.dev,
linux-kernel@vger.kernel.org,
Linus Torvalds <torvalds@linux-foundation.org>,
kuba@kernel.org
Subject: Re: [PATCH v3] iommu: Optimise PCI SAC address trick
Date: Thu, 13 Apr 2023 11:16:22 +0100 [thread overview]
Message-ID: <02eebcda-f60e-f8ed-7057-cf293d15a173@oracle.com> (raw)
In-Reply-To: <40141c33-243c-5da6-fbea-3122e47c7808@oracle.com>
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@huawei.com>
>> CC: Linus Torvalds <torvalds@linux-foundation.org>
>> Signed-off-by: Robin Murphy <robin.murphy@arm.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@oracle.com>
Is there any chance that this can be picked up?
I also saw that it fixed an issue for Jakub (cc'ed) recently.
next prev parent reply other threads:[~2023-04-13 10:17 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2023-01-18 17:26 [PATCH v3] iommu: Optimise PCI SAC address trick Robin Murphy
2023-01-20 11:33 ` John Garry
2023-04-13 10:16 ` John Garry [this message]
2023-04-13 10:44 ` Robin Murphy
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