[PATCH] x86/asm/entry/64: Improve the THREAD_INFO() macro explanation

[Ingo Molnar]

Explain the background, and add a real example.

Cc: Alexei Starovoitov <ast@xxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxxxxxx>
Cc: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxx>
Cc: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
Cc: Frederic Weisbecker <fweisbec@xxxxxxxxx>
Cc: H. Peter Anvin <hpa@xxxxxxxxx>
Cc: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Will Drewry <wad@xxxxxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>
---
 arch/x86/include/asm/thread_info.h | 27 +++++++++++++++++++++++----
 1 file changed, 23 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/thread_info.h b/arch/x86/include/asm/thread_info.h
index ad0ee3423da5..813dfbb867a7 100644
--- a/arch/x86/include/asm/thread_info.h
+++ b/arch/x86/include/asm/thread_info.h
@@ -206,10 +206,29 @@ static inline unsigned long current_stack_pointer(void)
 	_ASM_SUB $(THREAD_SIZE),reg ;
 
 /*
- * ASM operand which evaluates to thread_info address
- * if it is known that "reg" is exactly "off" bytes below stack top.
- * Example (fetch thread_info->fieldname):
- *  mov TI_fieldname+THREAD_INFO(reg, off),%eax
+ * ASM operand which evaluates to a 'thread_info' address of
+ * the current task, if it is known that "reg" is exactly "off"
+ * bytes below the top of the stack currently.
+ *
+ * ( The kernel stack's size is known at build time, it is usually
+ *   2 or 4 pages, and the bottom  of the kernel stack contains
+ *   the thread_info structure. So to access the thread_info very
+ *   quickly from assembly code we can calculate down from the
+ *   top of the kernel stack to the bottom, using constant,
+ *   build-time calculations only. )
+ *
+ * For example, to fetch the current thread_info->flags value into %eax
+ * on x86-64 defconfig kernels, in syscall entry code where RSP is
+ * currently at exactly SIZEOF_PTREGS bytes away from the top of the
+ * stack:
+ *
+ *      mov TI_flags+THREAD_INFO(%rsp, SIZEOF_PTREGS), %eax
+ *
+ * will translate to:
+ *
+ *      8b 84 24 b8 c0 ff ff      mov    -0x3f48(%rsp), %eax
+ *
+ * which is below the current RSP by almost 16K.
  */
 #define THREAD_INFO(reg, off) ((off)-THREAD_SIZE)(reg)
 
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