Re: x86_64 and CX8

[Jason Thorpe <thorpej%me.com@localhost>]

> On Apr 14, 2020, at 9:34 AM, Manuel Bouyer <bouyer%antioche.eu.org@localhost> wrote:
> 
> hello,
> is someone aware of a x86_64 CPU which does *not* support cmpxchg8b ?
> The instruction seems mandatory since i686-class CPUs, and it seems that
> since windows XP cmpxchg8b is not optional.
> 
> in amd64/amd64/spl.S we have 2 implementations of spllower(), which seems
> overkill if all x86-64 CPUs supports cmpxchg8b ...

I would certainly be surprised if cmpxchg8b was missing from any x86_64 CPUs, but you never know.

Maybe the best thing to do is rewrite the amd64 flavors of that code to use cmpxchgq, which is guaranteed to always there on all x86_64 CPUs (and upon which we make assumptions about its presence all over the place)?

-- thorpej