linux-kernel - Re: [PATCH rc v5 1/4] iommu/arm-smmu-v3: Add update

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Message-ID: <20260108003646.GA537728@nvidia.com>
Date: Wed, 7 Jan 2026 20:36:46 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Will Deacon <will@...nel.org>
Cc: Nicolin Chen <nicolinc@...dia.com>, robin.murphy@....com,
	joro@...tes.org, linux-arm-kernel@...ts.infradead.org,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	skolothumtho@...dia.com, praan@...gle.com,
	xueshuai@...ux.alibaba.com, smostafa@...gle.com
Subject: Re: [PATCH rc v5 1/4] iommu/arm-smmu-v3: Add update_safe bits to fix
 STE update sequence

On Wed, Jan 07, 2026 at 09:20:06PM +0000, Will Deacon wrote:
> >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.h   |  2 ++
> >  .../iommu/arm/arm-smmu-v3/arm-smmu-v3-test.c  | 18 ++++++++++---
> >  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3.c   | 27 ++++++++++++++-----
> >  3 files changed, 37 insertions(+), 10 deletions(-)
> 
> Hmm. So this appears to ignore the safe bits entirely, whereas the
> rationale for the change is that going from {MEV,EATS} disabled to
> enabled is safe (which I agree with). 

The argument was it doesn't matter for either direction be it disabled
to enabled or vice versa, see my reply to Mustfa in the v4 posting:

https://lore.kernel.org/all/20251218180129.GA254720@nvidia.com/

> So what prevents an erroneous hitless STE update when going from
> {MEV,EATS} enabled to disabled after this change?

Nothing, it isn't erroneous.

Jason