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

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Message-ID: <20250918143650.GQ1326709@ziepe.ca>
Date: Thu, 18 Sep 2025 11:36:50 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Will Deacon <will@...nel.org>
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 Thu, Sep 18, 2025 at 11:26:50AM +0100, Will Deacon wrote:
> On Wed, Sep 17, 2025 at 12:59:31PM -0300, Jason Gunthorpe wrote:
> > On Wed, Sep 17, 2025 at 04:25:35PM +0100, Will Deacon wrote:
> > 
> > > Ah right, so the driver would unnecessarily issue CMOs for the structures
> > > that are just shared with the hypervisor. At least it's _functional_ that
> > > way, but I'm sure people will complain!
> > 
> > Yes, functional, why would anyone complain? STE and CD manipulation is
> > not fast path for anything?
> 
> Won't it also apply to cmdq insertion?

Oh, changing CMDQ wasn't on my mind..

Yeah, OK I don't know what the performance delta would be like there.

However, to get peak performance out of pkvm we really do want the
SMMU driver to write CMDQ as cachable, pkvm to read it as cachable and
then copy it to a non-cachable HW queue.

Otherwise pkvm will be issuing CMOs on fast paths :\

If we convert the slow speed stuff, STE, CD, Fault to do CMOs then we
could make a fairly small change for pkvm mode to force the guest CMDQ
to be cachable without CMO. Some special feature triggered by pkvm
detection during probe.

Jason