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

[Andy Lutomirski]

Acked-by: Andy Lutomirski <luto@xxxxxxxxxx>

On Tue, Mar 24, 2015 at 11:50 AM, Denys Vlasenko <dvlasenk@xxxxxxxxxx> wrote:
> On 03/24/2015 07:43 PM, Ingo Molnar wrote:
>> From 0229a184997a7d4ad4398ee3ac2f5ae78c1c1a03 Mon Sep 17 00:00:00 2001
>> From: Ingo Molnar <mingo@xxxxxxxxxx>
>> 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@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)
>
> Acked-by: Denys Vlasenko <dvlasenk@xxxxxxxxxx>
>



-- 
Andy Lutomirski
AMA Capital Management, LLC
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