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    SubjectRe: [BUG] x86 kenel won't boot under Virtual PC
    Linus Torvalds wrote:
    >>>
    >> Yes, it does:
    >>
    >> /* We need to decide which NOP sequence to use for 32bit and
    >> 64bit. When -mtune= is used:
    >>
    >> 1. For PROCESSOR_I386, PROCESSOR_I486, PROCESSOR_PENTIUM and
    >> PROCESSOR_GENERIC32, f32_patt will be used.
    >> 2. For PROCESSOR_PENTIUMPRO, PROCESSOR_PENTIUM4, PROCESSOR_NOCONA,
    >> PROCESSOR_CORE, PROCESSOR_CORE2, and PROCESSOR_GENERIC64,
    >> alt_long_patt will be used.
    >> 3. For PROCESSOR_ATHLON, PROCESSOR_K6, PROCESSOR_K8 and
    >> PROCESSOR_AMDFAM10, alt_short_patt will be used.
    >>
    >> When -mtune= isn't used, alt_long_patt will be used if
    >> cpu_arch_isa_flags has Cpu686. Otherwise, f32_patt will
    >> be used.
    >>
    >> "alt_long_patt" uses NOPL and its variants.
    >
    > That is A TOTAL PIECE OF SH*T, and against gcc's own documentation.
    >
    > "-mtune=x" is very much defined to be a performance _tuning_ option, not
    > an "architectural" option.
    >
    > Quite frankly, this is a gcc bug. Plain and simple.
    >

    OK, digging some more into this garbage...

    Apparently the situation isn't quite as dire as it first seems.
    At least gcc 4.3.0 doesn't actually pass -mtune= to gas; it just drops
    the option on the floor. This means this bug isn't manifest when
    calling from gcc (as opposed to invoking as directly.)

    However, I would still like to push the following patch to be on the
    safe side, ok?

    -hpa

    From 0fbbf1008e5177677dbdb6292d94ec40df965d82 Mon Sep 17 00:00:00 2001
    From: H. Peter Anvin <hpa@zytor.com>
    Date: Mon, 8 Sep 2008 12:01:48 -0700
    Subject: [PATCH] x86: prevent binutils from being "smart" and generating NOPLs for us

    binutils, contrary to documented behaviour, will generate long NOPs (a
    P6-or-higher instruction which is broken on at least some VIA chips,
    Virtual PC/Virtual Server, and some versions of Qemu) depending on the
    -mtune= option, which is not supposed to change architectural
    behaviour.

    Pass an explicit override to the assembler, in case ends up passing
    the -mtune= parameter to gas (gcc 4.3.0 does not appear to.)

    Signed-off-by: H. Peter Anvin <hpa@zytor.com>
    ---
    arch/x86/Makefile_32.cpu | 5 +++++
    1 files changed, 5 insertions(+), 0 deletions(-)

    diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu
    index e372b58..b72b4f7 100644
    --- a/arch/x86/Makefile_32.cpu
    +++ b/arch/x86/Makefile_32.cpu
    @@ -45,3 +45,8 @@ cflags-$(CONFIG_MGEODEGX1) += -march=pentium-mmx
    # cpu entries
    cflags-$(CONFIG_X86_GENERIC) += $(call tune,generic,$(call tune,i686))

    +# Bug fix for binutils: this option is required in order to keep
    +# binutils from generating NOPL instructions against our will.
    +ifneq ($(CONFIG_X86_P6_NOP),y)
    +cflags-y += $(call cc-option,-Wa$(comma)-mtune=generic32,)
    +endif
    --
    1.5.5.1
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