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

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Message-ID: <aNvTME-Fmw8lpT25@google.com>
Date: Tue, 30 Sep 2025 12:55:12 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jason Gunthorpe <jgg@...pe.ca>
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 Tue, Sep 30, 2025 at 09:38:39AM -0300, Jason Gunthorpe wrote:
> 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.

I see, I think you mean[1], thanks for pointing it, I think we have to
keep things as is.

Thanks,
Mostafa

[1] https://lore.kernel.org/all/8f912671-f1d9-4f73-9c1d-e39938bfc09f@arm.com/

> 
> Jason