linux-kernel - Re: [PATCH 1/3] iommu/vt-d: Use 128-bit atomic updates for context entries

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Message-ID: <20260116143331.GJ961588@nvidia.com>
Date: Fri, 16 Jan 2026 10:33:31 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: 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>,
	Samiullah Khawaja <skhawaja@...gle.com>,
	"iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/3] iommu/vt-d: Use 128-bit atomic updates for context
 entries

On Fri, Jan 16, 2026 at 05:19:52AM +0000, Tian, Kevin wrote:
> btw the way that VT-d spec describes it as "software MUST use
> 128bit..." highly suggest that it's either as AMD or with exception
> only on very early generations. Another exception is VM which
> may have hardware supporting 128bit but vcpu model doesn't,
> but I doubt it's a meaningful configuration nowadays...

The VMs is an interesting point..

I think with a bit of fiddling the entry_set mechanism could fall back
to 64 bit operation automatically, and that would handle the
compatability needs. It would be much less hitless in such systems but
who cares?

Jason