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

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Message-ID: <20250915163858.GK882933@ziepe.ca>
Date: Mon, 15 Sep 2025 13:38:58 -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 Fri, Sep 12, 2025 at 03:18:08PM +0100, Will Deacon wrote:
> Ideally, the data structures that are shadowed by the hypervisor would
> be mapped as normal-WB cacheable in both the host and the hypervisor so
> we don't have to worry about coherency and we get the performance
> benefits from the caches. Indeed, I think that's how you've mapped
> 'host_cmdq' above _however_ I sadly don't think we can do that if the
> actual SMMU hardware isn't coherent.

That seems like the right conclusion to me, pkvm should not be mapping
as cachable unless it knows the IORT/IDR is marked as coherent.

This is actually something I want to fix in the SMMU driver, it should
always be allocating cachable memory and using
dma_sync_single_for_device() instead of non-cachable DMA coherent
allocations. (Or perhaps better is to use
iommu_pages_flush_incoherent())

I'm hearing about an interesting use case where we'd want to tell the
SMMU to walk STEs non-cachable even if the HW is capable to do
cachable. Apparently in some SOCs it gives better isochronous
properties for realtime DMA.

IMHO for this series at this point pkvm should just require a coherent
SMMU until the above revisions happen.

Jason