Re: [PATCH 0/5] iommu: Some IOVA code reorganisation

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On 2021-10-04 12:44, Will Deacon wrote:
On Fri, Sep 24, 2021 at 06:01:52PM +0800, John Garry wrote:
The IOVA domain structure is a bit overloaded, holding:
- IOVA tree management
- FQ control
- IOVA rcache memories

Indeed only a couple of IOVA users use the rcache, and only dma-iommu.c
uses the FQ feature.

This series separates out that structure. In addition, it moves the FQ
code into dma-iommu.c . This is not strictly necessary, but it does make
it easier for the FQ domain lookup the rcache domain.

The rcache code stays where it is, as it may be reworked in future, so
there is not much point in relocating and then discarding.

This topic was initially discussed and suggested (I think) by Robin here:
https://lore.kernel.org/linux-iommu/1d06eda1-9961-d023-f5e7-fe87e768f067@xxxxxxx/

It would be useful to have Robin's Ack on patches 2-4. The implementation
looks straightforward to me, but the thread above isn't very clear about
what is being suggested.

FWIW I actually got about half-way through writing my own equivalent of patches 2-3, except tackling it from the other direction - simplifying the FQ code *before* moving whatever was left to iommu-dma, then I got side-tracked trying to make io-pgtable use that freelist properly, and then I've been on holiday the last 2 weeks. I've got other things to catch up on first but I'll try to get to this later this week.

To play devil's advocate: there aren't many direct users of the iovad code:
either they'll die out entirely (and everybody will use the dma-iommu code)
and it's fine having the flush queue code where it is, or we'll get more
users and the likelihood of somebody else wanting flush queues increases.

I think the FQ code is mostly just here as a historical artefact, since the IOVA allocator was the only thing common to the Intel and AMD DMA ops when the common FQ implementation was factored out of those, so although it's essentially orthogonal it was still related enough that it was an easy place to stick it.

Cheers,
Robin.
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