linux-kernel - Re: [PATCH v10 05/10] iommu/amd: Introduce helper function to update 256-bit DTE

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Message-ID: <20241113142807.GJ35230@nvidia.com>
Date: Wed, 13 Nov 2024 10:28:07 -0400
From: Jason Gunthorpe <jgg@...dia.com>
To: Uros Bizjak <ubizjak@...il.com>
Cc: Arnd Bergmann <arnd@...db.de>,
	Suravee Suthikulpanit <suravee.suthikulpanit@....com>,
	linux-kernel@...r.kernel.org, iommu@...ts.linux.dev,
	Joerg Roedel <joro@...tes.org>, Robin Murphy <robin.murphy@....com>,
	vasant.hegde@....com, Linux-Arch <linux-arch@...r.kernel.org>,
	Kevin Tian <kevin.tian@...el.com>, jon.grimm@....com,
	santosh.shukla@....com, pandoh@...gle.com, kumaranand@...gle.com
Subject: Re: [PATCH v10 05/10] iommu/amd: Introduce helper function to update
 256-bit DTE

On Wed, Nov 13, 2024 at 03:14:09PM +0100, Uros Bizjak wrote:
> > > Even without atomicity guarantee, __READ_ONCE() still prevents the
> > > compiler from performing unwanted optimizations (please see the first
> > > comment in include/asm-generic/rwonce.h) and unwanted reordering of
> > > reads and writes when this function is inlined. This macro does cast
> > > the read to volatile, but IMO it is much more readable to use
> > > __READ_ONCE() than volatile qualifier.
> >
> > Yes it does, but please explain to me what "unwanted reordering" is
> > allowed here?
> 
> It is a static function that will be inlined by the compiler
> somewhere, so "unwanted reordering" depends on where it will be
> inlined. *IF* it will be called from safe code, then this limitation
> for the compiler can be lifted.

As long as the values are read within the spinlock the order does not
matter. READ_ONCE() is not required to contain reads within spinlocks.

Jason