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

From: Jamie Lokier (jamie_at_shareable.org)
Date: 09/30/03

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    Date:	Tue, 30 Sep 2003 17:54:50 +0100
To: Dave Jones <davej@redhat.com>, akpm@osdl.org, torvalds@osdl.org, linux-kernel@vger.kernel.org, richard.brunner@amd.com

    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
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