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

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Message-ID:
 <CH3PR12MB75480D5FB0B0027AAAA48CD5ABF8A@CH3PR12MB7548.namprd12.prod.outlook.com>
Date: Fri, 31 Oct 2025 12:49:07 +0000
From: Shameer Kolothum <skolothumtho@...dia.com>
To: Nicolin Chen <nicolinc@...dia.com>, Jason Gunthorpe <jgg@...dia.com>,
	"will@...nel.org" <will@...nel.org>
CC: "robin.murphy@....com" <robin.murphy@....com>, "joro@...tes.org"
	<joro@...tes.org>, "kevin.tian@...el.com" <kevin.tian@...el.com>,
	"praan@...gle.com" <praan@...gle.com>, "linux-arm-kernel@...ts.infradead.org"
	<linux-arm-kernel@...ts.infradead.org>, "iommu@...ts.linux.dev"
	<iommu@...ts.linux.dev>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] iommu/arm-smmu-v3-iommufd: Allow attaching nested domain
 for GBPA cases



> -----Original Message-----
> From: Nicolin Chen <nicolinc@...dia.com>
> Sent: 24 October 2025 05:06
> To: Jason Gunthorpe <jgg@...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; Shameer Kolothum
> <skolothumtho@...dia.com>
> Subject: [PATCH] iommu/arm-smmu-v3-iommufd: Allow attaching nested
> domain for GBPA cases
> 
> 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.
> 
> Add an additional condition in arm_smmu_attach_prepare_vmaster(), to
> allow
> a bypass or abort vSTE working for a GBPA setup. And do not forget to skip
> vsid=0 when reporting vevents, since the guest SMMU in this setup will not
> report event anyway.
> 
> Update the uAPI doc accordingly.
> 
> Signed-off-by: Nicolin Chen <nicolinc@...dia.com>

Looks good to me. Also tested with Qemu SMMuv3 accel v5 series
here,
https://lore.kernel.org/qemu-devel/20251031105005.24618-1-skolothumtho@nvidia.com/

Tested-by: Shameer Kolothum <skolothumtho@...dia.com>

Thanks,
Shameer