linux-kernel - Re: [PATCH 1/1] iommu/vt-d: Fix missed device TLB cache tag

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Date: Thu, 20 Jun 2024 11:08:15 -0300
From: Jason Gunthorpe <jgg@...pe.ca>
To: Vasant Hegde <vasant.hegde@....com>
Cc: Baolu Lu <baolu.lu@...ux.intel.com>,
	"Tian, Kevin" <kevin.tian@...el.com>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Jacek Lawrynowicz <jacek.lawrynowicz@...ux.intel.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/1] iommu/vt-d: Fix missed device TLB cache tag

On Thu, Jun 20, 2024 at 04:19:46PM +0530, Vasant Hegde wrote:
> >>>> seems that for all domain attaches above is coded in a wrong order
> >>>> as ats is enabled after the cache tag is assigned.
> >>> Yes, exactly. But simply changing the order isn't future-proof,
> >>> considering ATS control will eventually be moved out of iommu drivers.
> >> [Unrelated to this patch]
> >>
> >> You mean ATS setup will be moved to individual device driver? Is there any
> >> reason for that?
> > 
> > Not exactly to individual device drivers, but it should be out of the
> > iommu drivers.
> > 
> > https://lore.kernel.org/linux-iommu/BL1PR12MB51441FC4303BD0442EDB7A9CF7FFA@BL1PR12MB5144.namprd12.prod.outlook.com/
> 
> Got it. Thanks.
> 
> I remember of this discussion. May be we can provide API from IOMMU driver so
> that individual driver can enable/disable ATS (like iommu_dev_enable_feature()).

But I have a feeling if we do that it should be done by re-attaching
the domain.

For instance if you look at how I structued SMMUv3, the ATSness is an
effective property of the domain type and ATS switches on and off
dynamically already.

Having an additional input to domain attach "inhibit ats", as a flag
would be all the support the driver would need to provide for the core
code to manage this with some kind of global policy.

I would suggest to steer VTD in that direction too and make the ATS
enable be done on domain attach, and put the first ATS enable in
attach, not in probe. The logic in smmuv3 would apply just as well to
VTD, though you'd need the hitless update logic too :)

Jason