Re: [PATCH 1/5] x86: change THREAD_INFO definition to not depend on KERNEL_STACK_OFFSET

[Ingo Molnar <mingo@kernel.org>]

* Denys Vlasenko <dvlasenk@redhat.com> wrote:

> This changes THREAD_INFO definition and all its callsites
> so that they do not count stack position from
> (top of stack - KERNEL_STACK_OFFSET), but from top of stack.
> 
> Semi-mysterious expressions THREAD_INFO(%rsp,RIP) - "why RIP??"
> are now replaced by more logical THREAD_INFO(%rsp,SIZEOF_PTREGS) -
> "calculate thread_info's address using information that
> rsp is SIZEOF_PTREGS bytes below top of stack".
> 
> While at it, replace "(off)-THREAD_SIZE(reg)" with equivalent
> "((off)-THREAD_SIZE)(reg)". The form without parentheses
> falsely looks like we invoke THREAD_SIZE() macro.
> 
> Improve comment atop THREAD_INFO macro definition.
> 
> This patch does not change generated code (verified by objdump).

> --- a/arch/x86/include/asm/thread_info.h
> +++ b/arch/x86/include/asm/thread_info.h
> @@ -207,10 +207,12 @@ static inline unsigned long current_stack_pointer(void)
>  	_ASM_SUB $(THREAD_SIZE-KERNEL_STACK_OFFSET),reg ;
>  
>  /*
> - * Same if PER_CPU_VAR(kernel_stack) is, perhaps with some offset, already in
> - * a certain register (to be used in assembler memory operands).
> + * 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
>   */
> -#define THREAD_INFO(reg, off) KERNEL_STACK_OFFSET+(off)-THREAD_SIZE(reg)
> +#define THREAD_INFO(reg, off) ((off)-THREAD_SIZE)(reg)

We need more assembly hackers, so I have improved this still somewhat 
cryptic comment to:

/*
 * 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:
 *
 *      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)

Agreed?

Thanks,

	Ingo