Re: [RFC: DMA_PMD 00/22] DMA_PMD: PMD_SIZE-backed IO buffer pools

[Luigi Rizzo]

On Mon, Oct 12, 2026 at 3:51 PM David Hildenbrand (Arm)
<david@xxxxxxxxxx> wrote:
>
> On 10/3/26 23:22, Luigi Rizzo wrote:
> > Here is a subsystem called DMA_PMD on which I would like feedback on
> > architecture, possible enhancements, or kernel components that could be
> > reused to avoid duplication.
>
> Here is a feedback to a subsystem called DMA_PMD ;)
>
> Why is basing this on PMDs (and embedding it in the name) the right approach?
>
> On architctures with large PMDs (512 MiB on arm64 with 64k), would we really
> want to use 512 MiB pages, or rather something smaller (e.g., 2 MiB) that might
> possibly also have TLB benefits (cont-pte)?

I had to call it something... an earlier prototype used the name IOMMU_2M,
but I didn't want to suggest that the design was limited to 2M (it is not).

Aside from the name, the _current_ prototype restricts to 2M allocations,
exactly because too large units are expensive and wasteful.

In the code, decoupling the allocation size from the PMD size is trivial.

To enjoy the reduced IOTLB pressure the architecture needs to support
IOMMU leaves with the correct size. Looks like ARM supports a
"Contiguous" hint, but I am not sure how to tell if the hint is honored
(ie the IOTLB actually uses one entry for the full contiguous range)
other than measuring.

cheers
luigi