Immediate values provide a way to use dynamic code patching to update variables

sitting within the instruction stream. It saves caches lines normally used by

static read mostly variables. Enable it by default, but let users disable it

through the EMBEDDED menu with the "Disable immediate values" submenu entry.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>

CC: Adrian Bunk <bunk@stusta.de>

CC: Andi Kleen <andi@firstfloor.org>

CC: Alexey Dobriyan <adobriyan@gmail.com>

CC: Christoph Hellwig <hch@infradead.org>

---

 init/Kconfig |   21 +++++++++++++++++++++

 1 file changed, 21 insertions(+)
Index: linux-2.6-lttng/init/Kconfig

===================================================================

--- linux-2.6-lttng.orig/init/Kconfig	2007-09-04 11:53:59.000000000 -0400

+++ linux-2.6-lttng/init/Kconfig	2007-09-04 12:12:03.000000000 -0400

@@ -407,6 +407,17 @@ config CC_OPTIMIZE_FOR_SIZE

 config SYSCTL

 	bool
+config IMMEDIATE

+	default y if !DISABLE_IMMEDIATE

+	depends on X86_32 || PPC || PPC64

+	bool

+	help

+	  Immediate values are used as read mostly variables that are rarely

+	  updated. They use code patching to modify the values inscribed in the

+	  instruction stream. It provides a way to save precious cache lines

+	  that would otherwise have to be used by these variables. Can be

+	  disabled through the EMBEDDED menu.

+

 menuconfig EMBEDDED

 	bool "Configure standard kernel features (for small systems)"

 	help

@@ -645,6 +656,16 @@ config PROC_KPAGEMAP

           information on page-level memory usage. Disabling this interface

           will reduce the size of the kernel by around 600 bytes.
+config DISABLE_IMMEDIATE

+	default y if EMBEDDED

+	bool "Disable immediate values" if EMBEDDED

+	depends on X86_32 || PPC || PPC64

+	help

+	  Disable code patching based immediate values for embedded systems. It

+	  consumes slightly more memory and requires to modify the instruction

+	  stream each time a variable is updated. S...
[ message continues ]

It's still unclear to me why DISABLE_IMMEDIATE is needed. It would
There are no such on x86 at least  And for other architectures like

PPC it would be better to have a high level CONFIG_READ_ONLY_TEXT

then that does the necessary changes internally.  But I'm not sure it's even

supporting r/o text.
-Andi

-

It is actually the default on any non embedded configuration. Do you
The idea here is to give embedded system developers incentives to

create an optimized immediate value header for their architecture. I

fear that if it is not trivial to disable when they need to use ROM to

put the kernel code (as kprobes is, meaning, with a single config

option), they will refuse to event think about including an optimized

immediate value header for their architecture.
And yes, having a CONFIG_READ_ONLY_TEXT makes sense, but it implies

menu dependencies with not only immediate values but also kprobes,

paravirt, alternatives, (am I missing others ?)
As long as we find a way for people to disable _all_ code patching in

their kernel, I'm happy with that. But since every existing code

patching mechanism can currently be disabled one by one, it makes sense

to do the same for the immediate values. Having a global

CONFIG_READ_ONLY_TEXT should IMHO come in a separate effort.
--

Mathieu Desnoyers

Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal

OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

-

I would prefer to not have any config options at all and let
#ifdef CONFIG_ARCH_SPECIFIC_READONLY

#include <asm-generic/generic-immediate.h>

#else

/* optimized implementation */

#endif
paravirt and alternatives are x86 only.
I don't think CONFIG_READ_ONLY_TEXT on x86 makes sense.
On other architectures they have to deal with kprobes, but they
You're clearly deep into overdesign territory here.
-Andi

-

Yeah, but then you would make immediate.o an obj-y (both for

kernel/Makefile and arch/*/kernel/Makefile) and it would be built

even on systems that would be configured not to use immediate values.
Therefore, it would compile-in unused code if we do as you propose,

which I am reluctant to do on embedded systems where code size matters.
--

Mathieu Desnoyers

Computer Engineering Ph.D. Student, Ecole Polytechnique de Montreal

OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F  BA06 3F25 A8FE 3BAE 9A68

-

You can stick a define into generic-immediate.h and put an ifndef

for that around immediate.c
Or the recently posted function garbage collection patches would

also take care of it.
-Andi

-