Re: [PATCH v2 06/11] selftests: Fix arm64 IO barriers to match kernel

[David Laight <david.laight.linux@gmail.com>]

On Fri, 29 May 2026 10:49:47 -0300
Jason Gunthorpe <jgg@nvidia.com> wrote:

> On Thu, May 28, 2026 at 06:13:26PM +0000, David Matlack wrote:
> 
> > Let's put these in tools/arch/arm64/include/asm/io.h so that the tools
> > headers are more aligned with the kernel headers, and so that the arm64
> > io.h overrides are done in the same way as the x86 overrides in
> > tools/arch/x86/include/asm/io.h.
> > 
> > Something like this (untested):  
> 
> Okay, the disassembly says it works:
> 
>     1db8:       ca080108        eor     x8, x8, x8
>     1dbc:       b5000008        cbnz    x8, 1dbc <readl+0x58>
>     1dc0:       f9000fe8        str     x8, [sp, #24]

That looks strange, I suspect the C didn't match any usual pattern.
Normally 'tmp' would get thrown away and 'v' would get kept.
But you seem to have discarded 'v' and written 'tmp' to stack.

I'm probably being stupid again, but how does that work?
The cpu can speculate straight through the control dependency into
the following instructions.
An 'eor x1, x8, x8' may not even have a data-dependency on x8.
(Most x86 cpus just generate a zero for the equivalent instruction.)

-- David

> 
> Jason
>