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

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Message-ID: <aV7OBrH-TDiGRblt@willie-the-truck>
Date: Wed, 7 Jan 2026 21:20:06 +0000
From: Will Deacon <will@...nel.org>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: robin.murphy@....com, jgg@...dia.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 Thu, Dec 18, 2025 at 01:41:56PM -0800, Nicolin Chen wrote:
> From: Jason Gunthorpe <jgg@...dia.com>
> 
> C_BAD_STE was observed when updating nested STE from an S1-bypass mode to
> an S1DSS-bypass mode. As both modes enabled S2, the used bit is slightly
> different than the normal S1-bypass and S1DSS-bypass modes. As a result,
> fields like MEV and EATS in S2's used list marked the word1 as a critical
> word that requested a STE.V=0. This breaks a hitless update.
> 
> However, both MEV and EATS aren't critical in terms of STE update. One
> controls the merge of the events and the other controls the ATS that is
> managed by the driver at the same time via pci_enable_ats().
> 
> Add an arm_smmu_get_ste_update_safe() to allow STE update algorithm to
> relax those fields, avoiding the STE update breakages.
> 
> After this change, entry_set has no caller checking its return value, so
> change it to void.
> 
> Note that this change is required by both MEV and EATS fields, which were
> introduced in different kernel versions. So add get_update_safe() first.
> MEV and EATS will be added to arm_smmu_get_ste_update_safe() separately.
> 
> Fixes: 1e8be08d1c91 ("iommu/arm-smmu-v3: Support IOMMU_DOMAIN_NESTED")
> Cc: stable@...r.kernel.org
> Signed-off-by: Jason Gunthorpe <jgg@...dia.com>
> Reviewed-by: Shuai Xue <xueshuai@...ux.alibaba.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  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). So what prevents an erroneous
hitless STE update when going from {MEV,EATS} enabled to disabled after
this change?

Will