linux-kernel - Re: [PATCH v5 04/27] iommu/io-pgtable-arm: Factor kernel specific code out

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Message-ID: <aTw2x1UlldJGVA4Y@google.com>
Date: Fri, 12 Dec 2025 15:37:43 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org,
	kvmarm@...ts.linux.dev, iommu@...ts.linux.dev,
	catalin.marinas@....com, will@...nel.org, maz@...nel.org,
	oliver.upton@...ux.dev, joey.gouly@....com, suzuki.poulose@....com,
	yuzenghui@...wei.com, joro@...tes.org, jean-philippe@...aro.org,
	praan@...gle.com, danielmentz@...gle.com, mark.rutland@....com,
	qperret@...gle.com, tabba@...gle.com
Subject: Re: [PATCH v5 04/27] iommu/io-pgtable-arm: Factor kernel specific
 code out

On Fri, Nov 28, 2025 at 12:45:41PM -0400, Jason Gunthorpe wrote:
> On Mon, Nov 17, 2025 at 06:47:51PM +0000, Mostafa Saleh wrote:
> > Some of the used APIs are only part of the kernel and are not
> > available in the hypervisor, factor those out:
> > - alloc/free memory
> 
> Why not provide the iommu-pages API for the hypervisor environment?
> 
> Same for virt_to_phys, that could be moved into an iommu-pages wrapper
> too..

I guess that's possible, but then we would have to stub dma_map/unmap,
which might a bit ugly. I will look more into it.

> 
> I want to change other parts of the driver to use iommu-pages in the
> long run as well, so putting the abstraction there is probably more
> valuable than this.
> 
> Also the genericpt stuff is merged, should you (eventually?) be making
> a pKVM hypervisor specific set of page table functions? Eg if all you
> are doing is mirroring the host stage 2 I think you can build
> something much more efficient...

Most of the code is re-used from the io-pgtable-arm, so when that is
converted to genericpt, it should get to the hypervisor also.
The only hypervisor specific logic is the memory allocation, virt/phys
conversion and CMOs, I can look more into painting those, we don’t have
to change much.

I have some plans to add support for sharing the CPU stage-2 page table,
which is going to be complicated as it requires changes to the core
hypervisor hyp/pgtable.c code, which will be in a subsequent series.
However, I reckon that we would keep the shadowing logic as not all
SMMUv3/platforms are capable of sharing page tables.

Thanks,
Mostafa

> 
> Jason
>