[PATCH 7/7] x86: simplify 32-bit instruction set selection

[Arnd Bergmann]

On Thu, Nov 5, 2026, at 21:17, H. Peter Anvin wrote:
> On November 5, 2026 8:34:38 AM PST, Arnd Bergmann <arnd at kernel.org> wrote:
>>From: Arnd Bergmann <arnd at arndb.de>
>>
>>The -march= compiler flags select both instruction set and tuning for
>>a particular CPU, which works well when building a kernel that can only
>>run on this one type, but has some counterintuitive effects because it
>>is not always clear which models are compatible.
>>
>>E.g. building for Geode LX results in a kernel that can work on all on
>>all 686-class CPUs but crashes on CPUs without the CMOV instructions.
>>Building a kernel for 686 could work on Geode LX and Crusoe but fails
>>because of the CPU generation check that detects these as 585-class.
...
>
> Crusoe (TM3xxx/TM5xxx) do have a handful of compiler/user space visible 
> items that are missing from i686, as I recall notably the long nop 
> instructions and *possibly* fcomi (I *think* we caught that one, but 
> I'm not 100% sure.)
>
> As a result, it reports correctly as family 5.

I don't know about fcomi, but the P6 long NOP hasn't been used
in 32-bit kernels since your 2008 commit 14469a8dd236 ("x86:
disable static NOPLs on 32 bits"). From that point, I believe
we effectively treated Geode LX and Crusoe as 686-compatible.

binutils apparently uses the same definition and emits long
nops for -march=pentiumpro but not for -march=i686 in modern
versions.

You are of course correct that this remains inconsistent:
a kernel with CONFIG_MCRUSOE is still built with -march=i686
and CONFIG_X86_MINIMUM_CPU_FAMILY=5, so it will fail to run
on 586-class CPUs without CMOV.

I'll fold in this patch to make a MCRUSOE kernel actually compatible
with all i586-class CPUs, unless you have a better idea:

diff --git a/arch/x86/Makefile_32.cpu b/arch/x86/Makefile_32.cpu
index 1735a5f95d33..9a3a1b1c2452 100644
--- a/arch/x86/Makefile_32.cpu
+++ b/arch/x86/Makefile_32.cpu
@@ -21,7 +21,7 @@ cflags-$(CONFIG_MK6)		+= -march=i586 $(call tune,k6)
 # Please note, that patches that add -march=athlon-xp and friends are pointless.
 # They make zero difference whatsosever to performance at this time.
 cflags-$(CONFIG_MK7)		+= -march=i686 $(call tune,athlon)
-cflags-$(CONFIG_MCRUSOE)	+= -march=i686 $(align)
+cflags-$(CONFIG_MCRUSOE)	+= -march=i586 $(call tune,i686) $(align)
 cflags-$(CONFIG_MEFFICEON)	+= -march=i686 $(call tune,pentium3) $(align)
 cflags-$(CONFIG_MCYRIXIII)	+= -march=i586 $(call tune,i486) $(align)
 cflags-$(CONFIG_MVIAC3_2)	+= -march=i686

     Arnd