linux-kernel - Re: [PATCH 0/2] SMMU v3 CMDQ fix and improvement

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Message-ID: <aPIhMGnzHiBkIEam@google.com>
Date: Fri, 17 Oct 2025 10:57:52 +0000
From: Mostafa Saleh <smostafa@...gle.com>
To: Jacob Pan <jacob.pan@...ux.microsoft.com>
Cc: linux-kernel@...r.kernel.org,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Will Deacon <will@...nel.org>, Jason Gunthorpe <jgg@...dia.com>,
	Robin Murphy <robin.murphy@....com>,
	Nicolin Chen <nicolinc@...dia.com>,
	Zhang Yu <zhangyu1@...ux.microsoft.com>,
	Jean Philippe-Brucker <jean-philippe@...aro.org>,
	Alexander Grest <Alexander.Grest@...rosoft.com>
Subject: Re: [PATCH 0/2] SMMU v3 CMDQ fix and improvement

On Wed, Sep 24, 2025 at 10:54:36AM -0700, Jacob Pan wrote:
> Hi Will et al,
> 
> These two patches are derived from testing SMMU driver with smaller CMDQ
> sizes where we see soft lockups.
> 
> This happens on HyperV emulated SMMU v3 as well as baremetal ARM servers
> with artificially reduced queue size and microbenchmark to stress test
> concurrency.

Is it possible to share what are the artificial sizes and does the HW/emulation
support range invalidation (IRD3.RIL)?

I'd expect it would be really hard to overwhelm the command queue, unless the
HW doesn't support range invalidation and/or the queue entries are close to
the number of CPUs.

Thanks,
Mostafa


> 
> Thanks,
> 
> Jacob 
> 
> 
> Alexander Grest (1):
>   iommu/arm-smmu-v3: Improve CMDQ lock fairness and efficiency
> 
> Jacob Pan (1):
>   iommu/arm-smmu-v3: Fix CMDQ timeout warning
> 
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c | 85 +++++++++------------
>  1 file changed, 35 insertions(+), 50 deletions(-)
> 
> -- 
> 2.43.0
>