linux-kernel - Re: [PATCH rc] iommu/arm-smmu-v3: Do not set disable

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Message-ID: <20260120002213.GY1134360@nvidia.com>
Date: Mon, 19 Jan 2026 20:22:13 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>
Cc: will@...nel.org, robin.murphy@....com, joro@...tes.org,
	praan@...gle.com, linux-arm-kernel@...ts.infradead.org,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH rc] iommu/arm-smmu-v3: Do not set disable_ats unless vSTE
 is Translate

On Wed, Jan 14, 2026 at 05:12:43PM -0800, Nicolin Chen wrote:
> A vSTE may have three configuration types: Abort, Bypass, and Translate.
> 
> An Abort vSTE wouldn't enable ATS, but the other two might.
> 
> It makes sense for a Transalte vSTE to rely on the guest vSTE.EATS field.
> 
> For a Bypass vSTE, it would end up with an S2-only physical STE, similar
> to an attachment to a regular S2 domain. However, the nested case always
> disables ATS following the Bypass vSTE, while the regular S2 case always
> enables ATS so long as arm_smmu_ats_supported(master) == true.
> 
> Note that ATS is needed for certain VM centric workloads and historically
> non-vSMMU cases have relied on this automatic enablement. So, having the
> nested case behave differently causes problems.
> 
> To fix that, add a condition to disable_ats, so that it might enable ATS
> for a Bypass vSTE, aligning with the regular S2 case.
> 
> Fixes: f27298a82ba0 ("iommu/arm-smmu-v3: Allow ATS for IOMMU_DOMAIN_NESTED")
> Cc: stable@...r.kernel.org
> Suggested-by: Jason Gunthorpe <jgg@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 4 +++-
>  1 file changed, 3 insertions(+), 1 deletion(-)

Reviewed-by: Jason Gunthorpe <jgg@...dia.com>

Jason