linux-kernel - Re: [PATCH v4 10/28] KVM: arm64: iommu: Shadow host stage-2 page table

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Message-ID: <20250930123839.GL2695987@ziepe.ca>
Date: Tue, 30 Sep 2025 09:38:39 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Mostafa Saleh <smostafa@...gle.com>
Cc: Will Deacon <will@...nel.org>, linux-kernel@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-arm-kernel@...ts.infradead.org,
	iommu@...ts.linux.dev, maz@...nel.org, oliver.upton@...ux.dev,
	joey.gouly@....com, suzuki.poulose@....com, yuzenghui@...wei.com,
	catalin.marinas@....com, robin.murphy@....com,
	jean-philippe@...aro.org, qperret@...gle.com, tabba@...gle.com,
	mark.rutland@....com, praan@...gle.com
Subject: Re: [PATCH v4 10/28] KVM: arm64: iommu: Shadow host stage-2 page
 table

On Mon, Sep 29, 2025 at 11:01:10AM +0000, Mostafa Saleh wrote:

> > If the SMMU is in stage-1 bypass, we still have the incoming memory
> > attributes from the transaction (modulo MTCFG which we shouldn't be
> > setting) and they should combine with the stage-2 attributes in roughly
> > the same way as the CPU, no?
> 
> Makes sense, we can remove that for now and map all stage-2 with
> IOMMU_CACHE. 

Robin was saying in another thread that the DMA API has to use
IOMMU_MMIO properly or it won't work.. I think what happens depends on
the SOC design.

Yes, the incoming attribute combines, but unlike the CPU which will
have per-page memory attributes in the S1, the DMA initiator will
almost always use the same memory attributes.

In other words, we cannot rely on the DMA initiator to indicate if the
underlying memory should be MMIO or CACHE like the CPU can.

I think you have to set CACHE/MMIO correctly here.

Jason