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

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Message-ID: <CAFgf54ocNybHR_Mp1+i7cff-jkeMUV1QxBCt04jnKxwfwnyUfQ@mail.gmail.com>
Date: Thu, 8 Jan 2026 13:27:30 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 8, 2026 at 12:59 PM Jason Gunthorpe <jgg@...pe.ca> wrote:
>
> 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.
>

I see, so basically, we add the check for IOMMU_PAGES_USE_DMA_API in
iommu_pages_flush_incoherent() with the DMA-API stuff and call it from
the io-pgtable-arm instead.

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

That makes sense, I will look it over and send another version if
Robin is Ok with it.

Thanks,
Mostafa

> Jason