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

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Message-ID: <BL1PR11MB5271B47577A94CE3780D8ABF8C87A@BL1PR11MB5271.namprd11.prod.outlook.com>
Date: Tue, 6 Jan 2026 07:48:50 +0000
From: "Tian, Kevin" <kevin.tian@...el.com>
To: Jason Gunthorpe <jgg@...pe.ca>, Dmytro Maluka <dmaluka@...omium.org>
CC: David Woodhouse <dwmw2@...radead.org>, Lu Baolu
	<baolu.lu@...ux.intel.com>, "iommu@...ts.linux.dev" <iommu@...ts.linux.dev>,
	Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>, Robin Murphy
	<robin.murphy@....com>, "linux-kernel@...r.kernel.org"
	<linux-kernel@...r.kernel.org>, "Vineeth Pillai (Google)"
	<vineeth@...byteword.org>, Aashish Sharma <aashish@...hishsharma.net>,
	Grzegorz Jaszczyk <jaszczyk@...omium.org>, "Dong, Chuanxiao"
	<chuanxiao.dong@...el.com>
Subject: RE: [PATCH v2 0/5] iommu/vt-d: Ensure memory ordering in context &
 root entry updates

> From: Jason Gunthorpe <jgg@...pe.ca>
> Sent: Tuesday, January 6, 2026 8:14 AM
> 
> 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.

yeah WRITE_ONCE() is not by definition to guarantee the ordering between
CPU and device.

lots of READ_ONCE()/WRITE_ONCE() in existing code are meaningless,
as 1) between CPUs there is already lock protection; 2) between CPU and
device it requires dma_wmb() to guarantee the order.

> 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.

+1