Linus Torvalds wrote:
> I have conflicting reports about the safety of "movb" from Intel.
> According to some people in there, "movb" is always safe, and there should
> not be any need for any config option at all.
>
> However, at the same time my original contact at intel was Andy Glew, who
> probably knows more about the ia32 core than anybody else I know. And Andy
> says that yes "movb" is legal, but that some very early P6 steppings may
> be buggy. And Andy is God.
That comment in <asm-i386/spinlock.h> is rather tantalising. It says
don't use "movb" because it doesn't work but gives no clues why.
I still have the thread where this was hashed out. And it seemed very
few people ended up understanding the precise reason for not using
"movb". Not me :-(
Which is unfortunate, because I am trying to develop a model for machine
reasoning about ia32 instruction sequences. To generate better code,
and to check it. Even in user space, these SMP subtleties are
important. And I want to analyse kernel code :-)
So...
> I'd hate to have a kernel that works 99% of the time but then has
> occasional problems on some very rare machines that are really hard to
> track down. But I'd _almost_ like to just make the movb the default and
> have a CONFIG_BROKEN_P6_ORDERING options for the very very special
> case.
Could you ask Andy Glew which steppings are broken in this regard? Is
there a documented erratum? They usually list the broken steppings. Is
there a reliable test for the problem, preferably a fast one that can
run at boot? (I doubt it).
thanks,
-- Jamie
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This archive was generated by hypermail 2b29 : Sat Apr 22 2000 - 11:34:44 EDT