[PATCH] i386: allow disabling X86_FEATURE_SEP at boot

Allow the x86 "sep" feature to be disabled at bootup.  This forces use of the
int80 vsyscall.  Mainly for testing or benchmarking the int80 vsyscall code.
Signed-off-by: Chuck Ebbert <76306.1226@compuserve.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

Documentation/kernel-parameters.txt

@@ -1008,7 +1008,9 @@ running once the system is up.
 			noexec=on: enable non-executable mappings (default)
 			noexec=off: disable nn-executable mappings
 
-	nofxsr		[BUGS=IA-32]
+	nofxsr		[BUGS=IA-32] Disables x86 floating point extended
+			register save and restore. The kernel will only save
+			legacy floating-point registers on task switch.
 
 	nohlt		[BUGS=ARM]
 
@@ -1053,6 +1055,8 @@ running once the system is up.
 
 	nosbagart	[IA-64]
 
+	nosep		[BUGS=IA-32] Disables x86 SYSENTER/SYSEXIT support.
+
 	nosmp		[SMP] Tells an SMP kernel to act as a UP kernel.
 
 	nosync		[HW,M68K] Disables sync negotiation for all devices.

arch/i386/kernel/cpu/common.c

@@ -26,8 +26,9 @@ DEFINE_PER_CPU(unsigned char, cpu_16bit_stack[CPU_16BIT_STACK_SIZE]);
 EXPORT_PER_CPU_SYMBOL(cpu_16bit_stack);
 
 static int cachesize_override __cpuinitdata = -1;
-static int disable_x86_fxsr __cpuinitdata = 0;
+static int disable_x86_fxsr __cpuinitdata;
 static int disable_x86_serial_nr __cpuinitdata = 1;
+static int disable_x86_sep __cpuinitdata;
 
 struct cpu_dev * cpu_devs[X86_VENDOR_NUM] = {};
 
@@ -187,6 +188,14 @@ static int __init x86_fxsr_setup(char * s)
 __setup("nofxsr", x86_fxsr_setup);
 
 
+static int __init x86_sep_setup(char * s)
+{
+	disable_x86_sep = 1;
+	return 1;
+}
+__setup("nosep", x86_sep_setup);
+
+
 /* Standard macro to see if a specific flag is changeable */
 static inline int flag_is_changeable_p(u32 flag)
 {
@@ -405,6 +414,10 @@ void __cpuinit identify_cpu(struct cpuinfo_x86 *c)
 		clear_bit(X86_FEATURE_XMM, c->x86_capability);
 	}
 
+	/* SEP disabled? */
+	if (disable_x86_sep)
+		clear_bit(X86_FEATURE_SEP, c->x86_capability);
+
 	if (disable_pse)
 		clear_bit(X86_FEATURE_PSE, c->x86_capability);