[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>

2 files changed, 19 insertions, 2 deletions

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index fc99075e0af4..880be3a30d8d 100644
--- a/Documentation/kernel-parameters.txt
+++ b/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.
diff --git a/arch/i386/kernel/cpu/common.c b/arch/i386/kernel/cpu/common.c
index f63dcfb16dad..bbfc278dfa49 100644
--- a/arch/i386/kernel/cpu/common.c
+++ b/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);