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

On Mon, Oct 12, 2026 at 03:51:39PM +0200, David Hildenbrand (Arm) wrote:

On 10/3/26 23:22, Luigi Rizzo wrote:

quoted

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)?

+1 I raised the same point in the LPC talk.
AFAIK, this series tries to solve both IOTLB pressure and CoCo bouncing
at the same time which might be too strict; as for CoCo there is no
actual IOMMU, so there is no sizing constraint.

Also to reduce bouncing, I think it is sufficient to use the SWIOTLB
instead of having a new pool + recycling, the tricky part is how to
abstract that to client drivers as this would be needed beyond SKB.

Thanks,
Mostafa

-- 
Cheers,

David