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

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Message-ID: <BN9PR11MB5276A95E69D761271953E9F38C3DA@BN9PR11MB5276.namprd11.prod.outlook.com>
Date: Fri, 22 Aug 2025 09:45:28 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...dia.com>
CC: Nicolin Chen <nicolinc@...dia.com>, "robin.murphy@....com"
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	<helgaas@...nel.org>, "etzhao1900@...il.com" <etzhao1900@...il.com>
Subject: RE: [PATCH v3 3/5] iommu: Add iommu_get_domain_for_dev_locked()
 helper

> From: Jason Gunthorpe <jgg@...dia.com>
> Sent: Thursday, August 21, 2025 9:14 PM
> 
> 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.
> 

Thanks for the background. btw is it technically impossible or just
not worth of the extra software complexity for no value? e.g. if
maintaining two page tables (S1/S2 formats) with exact same mappings,
does SMMU allow smooth transition between two modes w/o breaking
in-fly DMAs? but probably keeping two page tables in-sync in transition
is already a problem w/o proper locking in map/unmap...