linux-kernel - Re: [PATCH v2] iommu/arm-smmu-v3-iommufd: Allow attaching nested domain for GBPA cases

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Message-ID: <20251124193435.GL153257@nvidia.com>
Date: Mon, 24 Nov 2025 15:34:35 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>, will@...nel.org
Cc: robin.murphy@....com, joro@...tes.org, kevin.tian@...el.com,
	praan@...gle.com, linux-arm-kernel@...ts.infradead.org,
	iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
	skolothumtho@...dia.com
Subject: Re: [PATCH v2] iommu/arm-smmu-v3-iommufd: Allow attaching nested
 domain for GBPA cases

On Mon, Nov 03, 2025 at 09:27:55AM -0800, Nicolin Chen wrote:
> A vDEVICE has been a hard requirement for attaching a nested domain to the
> device. This makes sense when installing a guest STE, since a vSID must be
> present and given to the kernel during the vDEVICE allocation.
> 
> But, when CR0.SMMUEN is disabled, VM doesn't really need a vSID to program
> the vSMMU behavior as GBPA will take effect, in which case the vSTE in the
> nested domain could have carried the bypass or abort configuration in GBPA
> register. Thus, having such a hard requirement doesn't work well for GBPA.
> 
> Skip vmaster allocation in arm_smmu_attach_prepare_vmaster() for an abort
> or bypass vSTE. Note that device on this attachment won't report vevents.
> 
> Update the uAPI doc accordingly.
> 
> Tested-by: Shameer Kolothum <skolothumtho@...dia.com>
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>
> ---
> 
> Changelog
> v2
>  * Add Tested-by from Shameer
>  * Skip vmaster allocation instead of bypassing vsid=0
>  * Revise the uAPI doc to note a corner case when CR0.SMMUEN=1
> 
>  drivers/iommu/arm/arm-smmu-v3/arm-smmu-v3-iommufd.c | 13 ++++++++++++-
>  include/uapi/linux/iommufd.h                        |  9 +++++++++
>  2 files changed, 21 insertions(+), 1 deletion(-)

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

Will, I will grab this in a few days unless you want it in your tree

Thanks,
Jason