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

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Message-ID: <aV0exRxVUoOaJtVj@mitya-t14-2025>
Date: Tue, 6 Jan 2026 15:40:05 +0100
From: Dmytro Maluka <dmaluka@...omium.org>
To: "Tian, Kevin" <kevin.tian@...el.com>
Cc: Jason Gunthorpe <jgg@...pe.ca>, 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

On Tue, Jan 06, 2026 at 07:48:50AM +0000, Tian, Kevin wrote:
> yeah WRITE_ONCE() is not by definition to guarantee the ordering between
> CPU and device.

Yes, WRITE_ONCE is not about HW guarantess at all, it is about compiler
guarantess. And it is not about ordering, it is about compiler's
guarantee to store the given 64-bit value once, with one instruction.
But this compiler guarantee is exactly my point (see my last reply to
Jason).

> 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 I see it, those WRITE_ONCEs (maybe not READ_ONCEs) haven't been
meaningless (I mean, they have been actually useful) so long as we
haven't been using any barriers. Again, on x86, store ordering requires
just compiler ordering, and dma_wmb() is just a compiler barrier. So,
assuming this driver is only used on x86 (which is, well, true :)),
we are lacking even compiler barriers, but at least we have those
WRITE_ONCEs, which provide compiler ordering too (although only between
each other, not with any other memory accesses, but that seems enough
for our case).

And again, I agree it is not pretty to rely on arch-specific ordering
assumptions, and doing in-place updates via those context_xxx() and
pasid_xxx() helpers all over the place instead of updating whole entries
seems a strange choice. But that is how it was implemented 10 or so
years ago, and overhauling that hasn't been my goal.