linux-kernel - Re: [PATCH v2 0/5] iommu/vt-d: Ensure memory ordering in context & root entry updates

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Message-ID: <20260106001418.GK125261@ziepe.ca>
Date: Mon, 5 Jan 2026 20:14:18 -0400
From: Jason Gunthorpe <jgg@...pe.ca>
To: Dmytro Maluka <dmaluka@...omium.org>
Cc: David Woodhouse <dwmw2@...radead.org>,
	Lu Baolu <baolu.lu@...ux.intel.com>, iommu@...ts.linux.dev,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>, linux-kernel@...r.kernel.org,
	"Vineeth Pillai (Google)" <vineeth@...byteword.org>,
	Aashish Sharma <aashish@...hishsharma.net>,
	Grzegorz Jaszczyk <jaszczyk@...omium.org>,
	Chuanxiao Dong <chuanxiao.dong@...el.com>,
	Kevin Tian <kevin.tian@...el.com>
Subject: Re: [PATCH v2 0/5] iommu/vt-d: Ensure memory ordering in context &
 root entry updates

On Mon, Jan 05, 2026 at 09:05:36PM +0100, Dmytro Maluka wrote:
> > and WRITE_ONCE is pointless if the HW is promising not to
> > read it due to non-present.
> 
> As long as we use a barrier. And IIUC vice versa, if we use WRITE_ONCE
> for any updates, a barrier is not necessary (on x86). And WRITE_ONCE for
> any updates (for PASID entries) is what was already done before this
> series.

That is an x86 ism and it shouldn't be needlessly leaked into drivers.
As this is not performance it should have the portable flow:

   WRITE_ONCE(non-present)
   dma_wmb()
   <cmd to flush caches>

   [..]
   <writes to the entry>

   dma_wmb()
   WRITE_ONCE(present)

So, it seems to me like all you need here is the one line to add the
dma_wmb() prior to setting present.

> Although, perhaps even with a barrier, WRITE_ONCE is still desirable to
> prevent the compiler from doing strange but legal things that involve
> transiently setting and then clearing the present bit behind our back?
> (Not that I'm aware of any compilers doing that, but I'm no compiler
> expert.)

You do have to write once the present bit, but not the others. The
dma_wmb() will make sure the HW cannot observe other states when it
observes present.

Jason