linux-kernel - Re: [PATCH 3/3] iommu/vt-d: Rework hitless PASID entry replacement

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Message-ID: <CAAywjhREggcahuWbk-ZqRjoW4v51X-gKw1pdA9TAcp-uO_6Bfg@mail.gmail.com>
Date: Wed, 14 Jan 2026 10:51:02 -0800
From: Samiullah Khawaja <skhawaja@...gle.com>
To: Jason Gunthorpe <jgg@...dia.com>
Cc: "Tian, Kevin" <kevin.tian@...el.com>, Baolu Lu <baolu.lu@...ux.intel.com>, 
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, Robin Murphy <robin.murphy@....com>, 
	Dmytro Maluka <dmaluka@...omium.org>, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 3/3] iommu/vt-d: Rework hitless PASID entry replacement

On Wed, Jan 14, 2026 at 5:17 AM Jason Gunthorpe <jgg@...dia.com> wrote:
>
> On Wed, Jan 14, 2026 at 07:26:10AM +0000, Tian, Kevin wrote:
> > before cache is flushed, it may contain:
> >
> >  - entries tagged with old DID, with content loaded from old table
> >  - entries tagged with old DID, with content loaded from new table
> >  - entries tagged with new DID, with content loaded from new table
> >
> > Compared to 2nd-stage the only problematic one is old DID + new table.
> >
> > According to 6.2.1 (Tagging of Cached Translations), the root address
> > of page table is not used in tagging and DID-based invalidation will
> > flush all entries related to old DID (no matter it's from old or new table).
> >
> > Then it should just work!
>
> Unless the original domain is attached to another device, then you've
> corrupted the DID and corrupted IOTLB for the second innocent device
> that isn't changing translation.
>
> > p.s. Jason said that atomic size is 128bit on AMD and 64bit on ARM.
> > they both have DID concept and two page table pointers. So I assume
> > it's the same case on this front?
>
> Hmm, yeah, ARM has it worse you can't change any ASID/VMID
> concurrently with the table pointer.
>
> You could make a safe algorithm by allocating a temporary ID, moving
> the current entry to the temporary ID, moving to the new pointer,
> moving to the final ID, then flushing the tempoary ID.

Interesting Idea. I think this can be done.
>
> It avoids the cross device issue and your logic above would hold.
>
> Or maybe the case Samiullah is interested in should have the new
> domain adopt the original ID..

There is no immediate use case for first level, as I understand we use
second level for all our VM use cases. But looking at the code and
specs, the first level is used for pasid compatible domains (using
IOMMU_HWPT_ALLOC_PASID flag) and it will not be replaceable
hitlessly..
>
> Jason