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

[Denys Vlasenko <dvlasenk@redhat.com>]

On 03/24/2015 07:43 PM, Ingo Molnar wrote:
> From 0229a184997a7d4ad4398ee3ac2f5ae78c1c1a03 Mon Sep 17 00:00:00 2001
> From: Ingo Molnar <mingo@kernel.org>
> Date: Tue, 24 Mar 2015 18:57:13 +0100
> Subject: [PATCH] x86/asm/entry/64: Improve the THREAD_INFO() macro explanation
> 
> Explain the background, and add a real example.
> 
> Cc: Alexei Starovoitov <ast@plumgrid.com>
> Cc: Andy Lutomirski <luto@amacapital.net>
> Cc: Andy Lutomirski <luto@kernel.org>
> Cc: Borislav Petkov <bp@alien8.de>
> Cc: Borislav Petkov <bp@suse.de>
> Cc: Denys Vlasenko <dvlasenk@redhat.com>
> Cc: Frederic Weisbecker <fweisbec@gmail.com>
> Cc: H. Peter Anvin <hpa@zytor.com>
> Cc: Kees Cook <keescook@chromium.org>
> Cc: Linus Torvalds <torvalds@linux-foundation.org>
> Cc: Oleg Nesterov <oleg@redhat.com>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Will Drewry <wad@chromium.org>
> Signed-off-by: Ingo Molnar <mingo@kernel.org>
> ---
>  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)

Acked-by: Denys Vlasenko <dvlasenk@redhat.com>