Hey,

I've been installing the latest Linux kernel 2.6.27.7 from source tarballs and saw a weird thing over the process. The compile scripts seem to ignore the fact I've chosen Pentium-MMX at CPU selection in `make menuconfig` and imply gcc flags like -mno-mmx -mcpu=i686, only -march=pentium-mmx seems to be correct.

Here is the whole parameter list (log of the compile):

gcc -E -D__GENKSYMS__ -Wp,-MD,net/ieee80211/.ieee80211_module.o.d -nostdinc -isystem /usr/lib/gcc-lib/i486-linux/3.3.5/include -D__KERNEL__ -Iinclude -I/usr/src/linux-2.6.27.7/arch/x86/include -include include/linux/autoconf.h -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -fno-strict-aliasing -fno-common -Werror-implicit-function-declaration -O2 -m32 -msoft-float -mregparm=3 -freg-struct-return -mpreferred-stack-boundary=2 -march=pentium-mmx -mcpu=i686 -ffreestanding -DCONFIG_AS_CFI=1 -pipe -Wno-sign-compare -fno-asynchronous-unwind-tables -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -Iinclude/asm-x86/mach-default -fomit-frame-pointer -DMODULE -DKBUILD_STR(s)=#s -DKBUILD_BASENAME=KBUILD_STR(ieee80211_module) -DKBUILD_MODNAME=KBUILD_STR(ieee80211) net/ieee80211/ieee80211_module.c

/usr/lib/gcc-lib/i486-linux/3.3.5/cc1 -E -quiet -nostdinc -Iinclude -I/usr/src/linux-2.6.27.7/arch/x86/include -Iinclude/asm-x86/mach-default -D__GNUC__=3 -D__GNUC_MINOR__=3 -D__GNUC_PATCHLEVEL__=5 -D__GENKSYMS__ -D__KERNEL__ -DCONFIG_AS_CFI=1 -DMODULE -DKBUILD_STR(s)=#s -DKBUILD_BASENAME=KBUILD_STR(ieee80211_module) -DKBUILD_MODNAME=KBUILD_STR(ieee80211) -isystem /usr/lib/gcc-lib/i486-linux/3.3.5/include -include include/linux/autoconf.h -MD net/ieee80211/.ieee80211_module.o.d net/ieee80211/ieee80211_module.c -m32 -msoft-float -mregparm=3 -mpreferred-stack-boundary=2 -march=pentium-mmx -mcpu=i686 -mno-sse -mno-mmx -mno-sse2 -mno-3dnow -Wall -Wundef -Wstrict-prototypes -Wno-trigraphs -Werror-implicit-function-declaration -Wno-sign-compare -fno-strict-aliasing -fno-common -freg-struct-return -ffreestanding -fno-asynchronous-unwind-tables -fomit-frame-pointer -O2

Is this a known issue? Do the flags mean something else?

Thanks in advance
stringZ

I once had something similar when compiling a kernel (2.6.18.1) - its menuconfig seemed a bit broken...

I fixed this by editing the .config file in the kernel source folder by hand. You might need to do this too?

E.g . I did

vi .config

in the kernel source tree, searched for the parameter I wanted (it had to with DMA) and I set that parameter on.

Parameters that are commented / off usually have the form

#FORCE_DMA_BLOCKS

which means it is not set. To set it change it to

FORCE_DMA_BLOCKS=y

(Note the above is only for illustrative purposes - I don't think such a parameter even exists... in any version of the kernel!)

E. g. in your case look for the parameter that you want sent, and see if in the .config you menuconfig produced,if it IS, in fact, set. If it is NOT set, set it like detailed above... It was the only way I managed to get certain options set the way I wanted them set.

Maybe you can do this too this way??

Within the configuration file, there are a set of options that determine the target CPU-type and model. I would very strongly suggest that you make sure that the one correct option has been specified, and "beyond that, leave it alone." One can presume that the Kernel Developers (and the other Lesser Gods) knew what they were about...

This is the World of the Arcane Mysteries. Step inside, breathe just a little of the heady incense, and then get the hell out of there...

Could you show us the actual value in your .config file just to verify that you saved it with the right CPU value?

-march=pentium-mmx defines the CPU that the code will require to run.
-mcpu=i686 specifies the CPU that the code is to be optimized for. i686 is actually a synonym for generic and generates code that is not specifically optimized for any processor model, i.e., it should run well on all models.

-mno-mmx says that compiler-generated code must not use the new MMX registers. Only applications use MMX registers; kernel code is not allowed to use them because then the kernel would have to save and restore these registers every time a program makes a system call, which would be less efficient overall.

cladisch: okay, so this is what makes my old PENTIUM I more efficient instead of slowing it down, right?

Yes.

You can get code optimized specifically for your CPU by disabling CONFIG_X86_GENERIC.