Re: [PATCH] Mutilated form of Andi Kleen's AMD prefetch errata patch

[Jamie Lokier <jamie@shareable.org>]

Dave Jones wrote:
> >    - I don't see anything that prevents a PPro-compiled kernel from booting
> >      on a P5MMX with the F00F erratum.
> 
> Compiled with -m686 - Uses CMOV, won't boot.

Ok, not PPro, but with Processor Family set to K6, CYRIXIII, or any of
the 3 WINCHIP choices, it is compiled with -march=i585 and without F00F.

(Fixing that by adding F00F too all those non-Intel processors, just to
make sure non-F00F kernels crash with a cmov instruction is too subtle
for my taste.)

Anyway, it should complain about lack of cmov not crash :)

> 1. The splitting of X86_FEATURE_XMM into X86_FEATURE_XMM_PREFETCH and
>    X86_FEATURE_3DNOW_PREFETCH doesn't seem to really buy us anything
>    other than complication.

I once suggested turning off XMM entirely when prefetch is broken and
not fixed, but that resulted in a mild toasting, hence the extra
synthetic flag.

It's not really split: XMM_PREFETCH is an additional flag, on top of
XMM, which simply means Linux decided it's safe to use the prefetch
instruction.

The only reason it's a feature flag is because the "alternative" macro
needs one.

> +       /* Prefetch works ok? */
> +#ifndef CONFIG_X86_PREFETCH_FIXUP
> +       if (c->x86_vendor != X86_VENDOR_AMD || c->x86 < 6)
> +#endif
> +       {
> +               if (cpu_has(c, X86_FEATURE_XMM))
> +                       set_bit(X86_FEATURE_XMM_PREFETCH, c->x86_capability);
> +               if (cpu_has(c, X86_FEATURE_3DNOW))
> +                       set_bit(X86_FEATURE_3DNOW_PREFETCH, c->x86_capability);
> +       }
> 
> - If we haven't set CONFIG_X86_PREFETCH_FIXUP (say a P4 kernel), this
>   code path isn't taken, and we end up not doing prefetches on P4's too
>   as you're not setting X86_FEATURE_XMM_PREFETCH anywhere else, and apply_alternatives
>   leaves them as NOPs.
> - Newer C3s are 686's with prefetch, this nobbles them too.

Read the code again.  It does what you think it doesn't do, so to speak.

-- Jamie