[patch 16/26] Immediate Values - x86 Optimization

Mathieu Desnoyers
2008-01-24T20:43:15Z

[-- Attachment #1: immediate-values-x86-optimization.patch --]
[-- Type: text/plain, Size: 5092 bytes --]

x86 optimization of the immediate values which uses a movl with code patching
to set/unset the value used to populate the register used as variable source.

Changelog:
- Use text_poke_early with cr0 WP save/restore to patch the bypass. We are doing
non atomic writes to a code region only touched by us (nobody can execute it
since we are protected by the imv_mutex).
- Put imv_set and _imv_set in the architecture independent header.
- Use $0 instead of %2 with (0) operand.
- Add x86_64 support, ready for i386+x86_64 -> x86 merge.
- Use asm-x86/asm.h.

Ok, so the most flexible solution that I see, that should fit for both
x86 and x86_64 would be :
1 byte : "=q" : "a", "b", "c", or "d" register for the i386. For
x86-64 it is equivalent to "r" class (for 8-bit
instructions that do not use upper halves).
2, 4, 8 bytes : "=r" : A register operand is allowed provided that it is in a
general register.

- "Redux" immediate values : no need to put a breakpoint, therefore, no
need to know where the instruction starts. It's therefore OK to have a
REX prefix.

- Bugfix : 8 bytes 64 bits immediate value was declared as "4 bytes" in the
immediate structure.
- Change the immediate.c update code to support variable length opcodes.
- Vastly simplified, using a busy looping IPI with interrupts disabled.
Does not protect against NMI nor MCE.
- Pack the __imv section. Use smallest types required for size (char).
- Use imv_* instead of immediate_*.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
CC: Andi Kleen <ak@muc.de>
CC: "H. Peter Anvin" <hpa@zytor.com>
CC: Chuck Ebbert <cebbert@redhat.com>
CC: Christoph Hellwig <hch@infradead.org>
CC: Jeremy Fitzhardinge <jeremy@goop.org>
CC: Thomas Gleixner <tglx@linutronix.de>
CC: Ingo Molnar <mingo@redhat.com>
CC: Rusty Russell <rusty@rustcorp.com.au>
---
arch/x86/Kconfig | 1
include/asm-x86/immediate.h | 77 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 78 insertions(+)

Index: linux-2.6-lttng.mm/include/asm-x86/immediate.h
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng.mm/include/asm-x86/immediate.h 2008-01-24 14:30:09.000000000 -0500
@@ -0,0 +1,77 @@
+#ifndef _ASM_X86_IMMEDIATE_H
+#define _ASM_X86_IMMEDIATE_H
+
+/*
+ * Immediate values. x86 architecture optimizations.
+ *
+ * (C) Copyright 2006 Mathieu Desnoyers <mathieu.desnoyers@polymtl.ca>
+ *
+ * This file is released under the GPLv2.
+ * See the file COPYING for more details.
+ */
+
+#include <asm/asm.h>
+
+/**
+ * imv_read - read immediate variable
+ * @name: immediate value name
+ *
+ * Reads the value of @name.
+ * Optimized version of the immediate.
+ * Do not use in __init and __exit functions. Use _imv_read() instead.
+ * If size is bigger than the architecture long size, fall back on a memory
+ * read.
+ *
+ * Make sure to populate the initial static 64 bits opcode with a value
+ * what will generate an instruction with 8 bytes immediate value (not the REX.W
+ * prefixed one that loads a sign extended 32 bits immediate value in a r64
+ * register).
+ */
+#define imv_read(name) \
+ ({ \
+ __typeof__(name##__imv) value; \
+ BUILD_BUG_ON(sizeof(value) > 8); \
+ switch (sizeof(value)) { \
+ case 1: \
+ asm(".section __imv,\"a\",@progbits\n\t" \
+ _ASM_PTR "%c1, (3f)-%c2\n\t" \
+ ".byte %c2\n\t" \
+ ".previous\n\t" \
+ "mov $0,%0\n\t" \
+ "3:\n\t" \
+ : "=q" (value) \
+ : "i" (&name##__imv), \
+ "i" (sizeof(value))); \
+ break; \
+ case 2: \
+ case 4: \
+ asm(".section __imv,\"a\",@progbits\n\t" \
+ _ASM_PTR "%c1, (3f)-%c2\n\t" \
+ ".byte %c2\n\t" \
+ ".previous\n\t" \
+ "mov $0,%0\n\t" \
+ "3:\n\t" \
+ : "=r" (value) \
+ : "i" (&name##__imv), \
+ "i" (sizeof(value))); \
+ break; \
+ case 8: \
+ if (sizeof(long) < 8) { \
+ value = name##__imv; \
+ break; \
+ } \
+ asm(".section __imv,\"a\",@progbits\n\t" \
+ _ASM_PTR "%c1, (3f)-%c2\n\t" \
+ ".byte %c2\n\t" \
+ ".previous\n\t" \
+ "mov $0xFEFEFEFE01010101,%0\n\t" \
+ "3:\n\t" \
+ : "=r" (value) \
+ : "i" (&name##__imv), \
+ "i" (sizeof(value))); \
+ break; \
+ }; \
+ value; \
+ })
+
+#endif /* _ASM_X86_IMMEDIATE_H */
Index: linux-2.6-lttng.mm/arch/x86/Kconfig
===================================================================
--- linux-2.6-lttng.mm.orig/arch/x86/Kconfig 2008-01-24 14:18:34.000000000 -0500
+++ linux-2.6-lttng.mm/arch/x86/Kconfig 2008-01-24 14:30:09.000000000 -0500
@@ -20,6 +20,7 @@ config X86
def_bool y
select HAVE_OPROFILE
select HAVE_KPROBES
+ select HAVE_IMMEDIATE

config GENERIC_LOCKBREAK
def_bool n