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

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Message-ID: <aWd2KCSU6XTNmQ4R@mitya-t14-2025>
Date: Wed, 14 Jan 2026 11:55:36 +0100
From: Dmytro Maluka <dmaluka@...omium.org>
To: Baolu Lu <baolu.lu@...ux.intel.com>
Cc: Joerg Roedel <joro@...tes.org>, Will Deacon <will@...nel.org>,
	Robin Murphy <robin.murphy@....com>,
	Kevin Tian <kevin.tian@...el.com>, Jason Gunthorpe <jgg@...dia.com>,
	Samiullah Khawaja <skhawaja@...gle.com>, iommu@...ts.linux.dev,
	linux-kernel@...r.kernel.org,
	"Vineeth Pillai (Google)" <vineeth@...byteword.org>,
	Aashish Sharma <aashish@...hishsharma.net>
Subject: Re: [PATCH 1/3] iommu/vt-d: Use 128-bit atomic updates for context
 entries

On Wed, Jan 14, 2026 at 01:14:36PM +0800, Baolu Lu wrote:
> On 1/14/26 03:27, Dmytro Maluka wrote:
> > On Tue, Jan 13, 2026 at 11:00:46AM +0800, Lu Baolu wrote:
> > > +static __always_inline void intel_iommu_atomic128_set(u128 *ptr, u128 val)
> > > +{
> > > +	/*
> > > +	 * Use the cmpxchg16b instruction for 128-bit atomicity. As updates
> > > +	 * are serialized by a spinlock, we use the local (unlocked) variant
> > > +	 * to avoid unnecessary bus locking overhead.
> > > +	 */
> > > +	arch_cmpxchg128_local(ptr, *ptr, val);
> > Any reason why not cmpxchg128_local()? (except following the AMD driver)
> 
> Yes. This follows the AMD IOMMU driver. Both drivers use spin lock to
> synchronize the update of table entries. They only need the atomicity of
> the 128-bit instruction itself. So arch_cmpxchg128_local() works.

Yeah, but my question was merely: why use the raw arch_*() version, not
cmpxchg128_local() which is the same but also includes optional
kasan/kcsan instrumentation:

#define cmpxchg128_local(ptr, ...) \
({ \
	typeof(ptr) __ai_ptr = (ptr); \
	instrument_atomic_read_write(__ai_ptr, sizeof(*__ai_ptr)); \
	raw_cmpxchg128_local(__ai_ptr, __VA_ARGS__); \
})

IOW, why bypass this instrumentation?