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

[Mostafa Saleh <smostafa@google.com>]

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

+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
>