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

[Jason Gunthorpe]

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