Messages in this thread Patch in this message | | | Date | Mon, 08 Sep 2008 12:09:56 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [BUG] x86 kenel won't boot under Virtual PC |
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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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