x86/math-emu: Limit MATH_EMULATION to 486SX compatibles

The FPU emulation code is old and fragile in places, try to limit its
use to builds for CPUs that actually use it. As far as I can tell,
this is only true for i486sx compatibles, including the Cyrix 486SLC,
AMD Am486SX and ÉLAN SC410, UMC U5S amd DM&P VortexSX86, all of which
were relatively short-lived and got replaced with i486DX compatible
processors soon after introduction, though some of the embedded versions
remained available much longer.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Bill Metzenthen <billm@melbpc.org.au>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191001142344.1274185-2-arnd@arndb.de

arch/x86/Kconfig

@@ -1751,7 +1751,7 @@ config X86_RESERVE_LOW
 config MATH_EMULATION
 	bool
 	depends on MODIFY_LDT_SYSCALL
-	prompt "Math emulation" if X86_32
+	prompt "Math emulation" if X86_32 && (M486SX || MELAN)
 	---help---
 	  Linux can emulate a math coprocessor (used for floating point
 	  operations) if you don't have one. 486DX and Pentium processors have

arch/x86/Kconfig.cpu

@@ -50,12 +50,19 @@ choice
 	  See each option's help text for additional details. If you don't know
 	  what to do, choose "486".
 
+config M486SX
+	bool "486SX"
+	depends on X86_32
+	---help---
+	  Select this for an 486-class CPU without an FPU such as
+	  AMD/Cyrix/IBM/Intel SL/SLC/SLC2/SLC3/SX/SX2 and UMC U5S.
+
 config M486
-	bool "486"
+	bool "486DX"
 	depends on X86_32
 	---help---
 	  Select this for an 486-class CPU such as AMD/Cyrix/IBM/Intel
-	  486DX/DX2/DX4 or SL/SLC/SLC2/SLC3/SX/SX2 and UMC U5D or U5S.
+	  486DX/DX2/DX4 and UMC U5D.
 
 config M586
 	bool "586/K5/5x86/6x86/6x86MX"
@@ -312,20 +319,20 @@ config X86_L1_CACHE_SHIFT
 	int
 	default "7" if MPENTIUM4 || MPSC
 	default "6" if MK7 || MK8 || MPENTIUMM || MCORE2 || MATOM || MVIAC7 || X86_GENERIC || GENERIC_CPU
-	default "4" if MELAN || M486 || MGEODEGX1
+	default "4" if MELAN || M486SX || M486 || MGEODEGX1
 	default "5" if MWINCHIP3D || MWINCHIPC6 || MCRUSOE || MEFFICEON || MCYRIXIII || MK6 || MPENTIUMIII || MPENTIUMII || M686 || M586MMX || M586TSC || M586 || MVIAC3_2 || MGEODE_LX
 
 config X86_F00F_BUG
 	def_bool y
-	depends on M586MMX || M586TSC || M586 || M486
+	depends on M586MMX || M586TSC || M586 || M486SX || M486
 
 config X86_INVD_BUG
 	def_bool y
-	depends on M486
+	depends on M486SX || M486
 
 config X86_ALIGNMENT_16
 	def_bool y
-	depends on MWINCHIP3D || MWINCHIPC6 || MCYRIXIII || MELAN || MK6 || M586MMX || M586TSC || M586 || M486 || MVIAC3_2 || MGEODEGX1
+	depends on MWINCHIP3D || MWINCHIPC6 || MCYRIXIII || MELAN || MK6 || M586MMX || M586TSC || M586 || M486SX || M486 || MVIAC3_2 || MGEODEGX1
 
 config X86_INTEL_USERCOPY
 	def_bool y
@@ -378,7 +385,7 @@ config X86_MINIMUM_CPU_FAMILY
 
 config X86_DEBUGCTLMSR
 	def_bool y
-	depends on !(MK6 || MWINCHIPC6 || MWINCHIP3D || MCYRIXIII || M586MMX || M586TSC || M586 || M486) && !UML
+	depends on !(MK6 || MWINCHIPC6 || MWINCHIP3D || MCYRIXIII || M586MMX || M586TSC || M586 || M486SX || M486) && !UML
 
 menuconfig PROCESSOR_SELECT
 	bool "Supported processor vendors" if EXPERT
@@ -402,7 +409,7 @@ config CPU_SUP_INTEL
 config CPU_SUP_CYRIX_32
 	default y
 	bool "Support Cyrix processors" if PROCESSOR_SELECT
-	depends on M486 || M586 || M586TSC || M586MMX || (EXPERT && !64BIT)
+	depends on M486SX || M486 || M586 || M586TSC || M586MMX || (EXPERT && !64BIT)
 	---help---
 	  This enables detection, tunings and quirks for Cyrix processors
 
@@ -470,7 +477,7 @@ config CPU_SUP_TRANSMETA_32
 config CPU_SUP_UMC_32
 	default y
 	bool "Support UMC processors" if PROCESSOR_SELECT
-	depends on M486 || (EXPERT && !64BIT)
+	depends on M486SX || M486 || (EXPERT && !64BIT)
 	---help---
 	  This enables detection, tunings and quirks for UMC processors
 

arch/x86/Makefile_32.cpu

@@ -10,6 +10,7 @@ else
 tune		= $(call cc-option,-mcpu=$(1),$(2))
 endif
 
+cflags-$(CONFIG_M486SX)		+= -march=i486
 cflags-$(CONFIG_M486)		+= -march=i486
 cflags-$(CONFIG_M586)		+= -march=i586
 cflags-$(CONFIG_M586TSC)	+= -march=i586

arch/x86/include/asm/module.h

@@ -15,6 +15,8 @@ struct mod_arch_specific {
 
 #ifdef CONFIG_X86_64
 /* X86_64 does not define MODULE_PROC_FAMILY */
+#elif defined CONFIG_M486SX
+#define MODULE_PROC_FAMILY "486SX "
 #elif defined CONFIG_M486
 #define MODULE_PROC_FAMILY "486 "
 #elif defined CONFIG_M586