linux-kernel - Re: [PATCH v4 22/28] iommu/arm-smmu-v3-kvm: Emulate CMDQ for host

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Message-ID: <aMrNTny9jgZJd_Ef@willie-the-truck>
Date: Wed, 17 Sep 2025 16:01:34 +0100
From: Will Deacon <will@...nel.org>
To: Jason Gunthorpe <jgg@...pe.ca>
Cc: Mostafa Saleh <smostafa@...gle.com>, 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 22/28] iommu/arm-smmu-v3-kvm: Emulate CMDQ for host

On Wed, Sep 17, 2025 at 09:36:01AM -0300, Jason Gunthorpe wrote:
> On Tue, Sep 16, 2025 at 03:19:02PM +0000, Mostafa Saleh wrote:
> 
> > I think the fix for the problem Will mentioned is to just use CMOs
> > before accessing the host structures, so that should be simple.
> > If it turns to be more complicated, I am happy to drop the support
> > for non-coherent devices from this series and we can add it later.
> 
> I feel like it is easier/better to fix the driver to use cachable
> memory than to add CMOs to the pkvm side..

Hmm, but for non-coherent SMMU hardware (which sadly exists in
production), I don't think there's a way for firmware to tell the driver
that it needs to issue CMOs for the page-tables and the CDs but not the
other in-memory data structures (e.g. STEs). I suppose we could do it in
some pKVM-specific way, but then that's not really helping anybody else.

Will