x86: Fix booting with "no387 nofxsr"

Jesper Juhl reported that testing the software math-emulation by forcing
"no387" doesn't work on modern CPU's.

The reason was two-fold:
 - you also need to pass in "nofxsr" to make sure that we not only don't
   touch the old i387 legacy hardware, it also needs to disable the
   modern XMM/FXSR sequences
 - "nofxsr" didn't actually clear the capability bits immediately,
   leaving the early boot sequence still using FXSR until we got to
   the identify_cpu() stage.

This fixes the "nofxsr" flag to take effect immediately on the boot CPU.

Debugging by Randy Dunlap

Acked-by: Randy Dunlap <rdunlap@xenotime.net>
Cc: Jesper Juhl <jesper.juhl@gmail.com>
Cc: Andi Kleen <ak@suse.de>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

1 files changed, 9 insertions, 0 deletions

diff --git a/arch/i386/kernel/cpu/common.c b/arch/i386/kernel/cpu/common.c
index 2799baa..b2f24d5 100644
--- a/arch/i386/kernel/cpu/common.c
+++ b/arch/i386/kernel/cpu/common.c
@@ -184,7 +184,16 @@ static void __cpuinit get_cpu_vendor(struct cpuinfo_x86 *c, int early)
 
 static int __init x86_fxsr_setup(char * s)
 {
+	/* Tell all the other CPU's to not use it... */
 	disable_x86_fxsr = 1;
+
+	/*
+	 * ... and clear the bits early in the boot_cpu_data
+	 * so that the bootup process doesn't try to do this
+	 * either.
+	 */
+	clear_bit(X86_FEATURE_FXSR, boot_cpu_data.x86_capability);
+	clear_bit(X86_FEATURE_XMM, boot_cpu_data.x86_capability);
 	return 1;
 }
 __setup("nofxsr", x86_fxsr_setup);