linux-kernel - Re: [PATCH] iommu/io-pgtable-arm: Drop DMA API usage for CMOs

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Message-ID: <20260108125934.GB545276@ziepe.ca>
Date: Thu, 8 Jan 2026 08:59:34 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: iommu@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	linux-kernel@...r.kernel.org, robin.murphy@....com, will@...nel.org,
	joro@...tes.org
Subject: Re: [PATCH] iommu/io-pgtable-arm: Drop DMA API usage for CMOs

On Thu, Jan 08, 2026 at 11:38:46AM +0000, Mostafa Saleh wrote:
> Jason pointed out that the DMA-API calls are not really needed [2].
> 
> Looking more into this. Initially, the io-pgtable API let drivers
> do the CMOs using tlb::flush_pgtable() where drivers were using the
> DMA API (map/unmap_single) only to do CMOs as the low-level cache
> functions won’t be available for modules.

This isn't what I ment at all, the iommu-pages.h could do the flush
inside itself using the already existing
iommu_pages_flush_incoherent() instead of open coding the dma api
calls everwhere, and maybe that could directly call the arch function
on arm (as x86 does) which would be easier to implement in pkvm's
hypervisor.

Then it is reasonable to have a pkvm version of the iommu-pages api
for the page table and drive rto use.

Jason