Make sysenter support optional

This adds configurable sysenter support on x86. This saves about 5k on small systems.

$ size vmlinux-baseline vmlinux
   text    data     bss     dec     hex filename
2920821  523232  190652 3634705  377611 vmlinux-baseline
2920558  518376  190652 3629586  376212 vmlinux

Index: 2.6.15-misc/arch/i386/kernel/Makefile
===================================================================
@@ -31,7 +31,7 @@ obj-$(CONFIG_X86_NUMAQ)		+= numaq.o
 obj-$(CONFIG_X86_SUMMIT_NUMA)	+= summit.o
 obj-$(CONFIG_KPROBES)		+= kprobes.o
 obj-$(CONFIG_MODULES)		+= module.o
-obj-y				+= sysenter.o vsyscall.o
+obj-$(CONFIG_SYSENTER)		+= sysenter.o vsyscall.o
 obj-$(CONFIG_ACPI_SRAT) 	+= srat.o
 obj-$(CONFIG_HPET_TIMER) 	+= time_hpet.o
 obj-$(CONFIG_EFI) 		+= efi.o efi_stub.o

Index: 2.6.15-misc/include/asm-i386/elf.h
===================================================================
@@ -134,11 +134,13 @@ extern int dump_task_extended_fpu (struc
 #define VSYSCALL_ENTRY	((unsigned long) &__kernel_vsyscall)
 extern void __kernel_vsyscall;
 
+#ifdef CONFIG_SYSENTER
 #define ARCH_DLINFO						\
 do {								\
 		NEW_AUX_ENT(AT_SYSINFO,	VSYSCALL_ENTRY);	\
 		NEW_AUX_ENT(AT_SYSINFO_EHDR, VSYSCALL_BASE);	\
 } while (0)
+#endif
 
 /*
  * These macros parameterize elf_core_dump in fs/binfmt_elf.c to write out

Index: 2.6.15-misc/init/Kconfig
===================================================================
@@ -352,6 +352,14 @@ config VM86
           XFree86 to initialize some video cards via BIOS. Disabling this
           option saves about 6k.
 
+config SYSENTER
+	depends X86
+	default y
+	bool "Enable syscalls via sysenter" if EMBEDDED
+	help
+	  Disabling this feature removes sysenter handling as well as
+	  vsyscall fixmaps.
+ 
 config CC_OPTIMIZE_FOR_SIZE
 	bool "Optimize for size"
 	default y if ARM || H8300

Index: 2.6.15-misc/include/asm-i386/processor.h
===================================================================
@@ -709,8 +709,14 @@ extern void select_idle_routine(const st
 #define cache_line_size() (boot_cpu_data.x86_cache_alignment)
 
 extern unsigned long boot_option_idle_override;
+
+#ifdef CONFIG_SYSENTER
 extern void enable_sep_cpu(void);
 extern int sysenter_setup(void);
+#else
+#define enable_sep_cpu()
+static inline int sysenter_setup(void) { return 0; }
+#endif
 
 #ifdef CONFIG_MTRR
 extern void mtrr_ap_init(void);

Mathematics is the supreme nostalgia of our time.

You not only remove the sysenter support but also the vdso. And the later is a very bad idea. It is already today basically impossible to have reliable backtraces without the vdso and the unwind info it contains for signal handlers. And things can only get worse in future.
The magic heuristics in the compiler are not reliable. It's simply the
wrong face, this in a interface between the kernel and the libc, the compiler should not have such knowledge.

The vdso should be mandatory for all configurations.

➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

It's under CONFIG_EMBEDDED. Think uclibc. Think systems without interactive shells.

Mathematics is the supreme nostalgia of our time.

Interactive or not has absolutely nothing to do with this. And other libcs have the same issues wrt to backtraces. And finally, there is unfortunately no way to prevent somebody from using glibc on an "embedded" kernel and I have no interest in getting "bug reports" caused by this kind of user error.

➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

Ok, let me be explicit: think systems with absolutely no facility for recording or displaying a backtrace.

As far as I'm aware, uclibc has no vdso support, so it might as well not exist for systems using it.

Mathematics is the supreme nostalgia of our time.

You don't know much about unwinding, do you? The same information is needed for C++ exception handling, thread cancellation, etc. Now go on and tell me you don't need this either.

And I told you that the support which magically makes all this sometimes work without the unwind info in the vdso will sooner or later break. Then whatever other libc is out there has to get vdso support to be able to function correctly.

➧ Ulrich Drepper ➧ Red Hat, Inc. ➧ 444 Castro St ➧ Mountain View, CA ❖

On (presumbly small) embedded devices?
Please be serious.

Bernd

Firmix Software GmbH                   http://www.firmix.at/
mobil: +43 664 4416156                 fax: +43 1 7890849-55
          Embedded Linux Development and Services

note that this also turns off vsyscalls. Right now vsyscalls are mostly about sysenter support, but that's not fundamentally so. 4k of the 5k savings come from the turn-off-vsyscalls portion.

Ingo

Yes, this patch would more properly be CONFIG_VSYSCALL.

Mathematics is the supreme nostalgia of our time.