linux-kernel - Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev

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Message-ID: <20250821131426.GN802098@nvidia.com>
Date: Thu, 21 Aug 2025 10:14:26 -0300
From: Jason Gunthorpe <jgg@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Nicolin Chen <nicolinc@...dia.com>,
	"robin.murphy@....com" <robin.murphy@....com>,
	"joro@...tes.org" <joro@...tes.org>,
	"bhelgaas@...gle.com" <bhelgaas@...gle.com>,
	"will@...nel.org" <will@...nel.org>,
	"robin.clark@....qualcomm.com" <robin.clark@....qualcomm.com>,
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	"Liu, Yi L" <yi.l.liu@...el.com>,
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	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
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	"helgaas@...nel.org" <helgaas@...nel.org>,
	"etzhao1900@...il.com" <etzhao1900@...il.com>
Subject: Re: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked()
 helper

On Thu, Aug 21, 2025 at 08:11:05AM +0000, Tian, Kevin wrote:
> > From: Nicolin Chen <nicolinc@...dia.com>
> > Sent: Tuesday, August 19, 2025 1:23 AM
> > 
> > ... I found that in SMMUv3 driver, iommu_get_domain_for_dev() is
> > used to get the RID domain for an SVA domain:
> >     arm_smmu_set_pasid()
> >     arm_smmu_blocking_set_dev_pasid()
> > 
> > These two are already given an "old" (SVA) domain pointer, FWIW.
> > 
> > So, we may change to passing in the old domain as you suggested,
> > yet we still have to fix the iommu_get_domain_for_dev() in order
> > to reflect the RID domain correctly for the driver that calls it
> > (or even potentially) in some group->mutex locked context where
> > the RID domain might not be naturally passed in.
> > 
> 
> Out of curiosity.
> 
> arm_smmu_blocking_set_dev_pasid()
> 
> 	/*                              
> 	 * When the last user of the CD table goes away downgrade the STE back
> 	 * to a non-cd_table one.
> 	 */
> 	if (!arm_smmu_ssids_in_use(&master->cd_table)) {
> 		struct iommu_domain *sid_domain =
> 			iommu_get_domain_for_dev(master->dev);
> 
> 		if (sid_domain->type == IOMMU_DOMAIN_IDENTITY ||
> 		   sid_domain->type == IOMMU_DOMAIN_BLOCKED)
> 			sid_domain->ops->attach_dev(sid_domain, dev); 
> 	}
> 
> why cannot downgrade apply to the case where the RID is attached to
> a DMA domain?

If the RID is a PAGING domain then it must be a S1 paging domain and there is
no downgrade possible.

It is impossible for the RID to be a S2 paging domain while ssids are
in use.

Jason