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per_cpu宏在linux kernel中很常见,此处结合代码简析一下per_cpu宏实现的功能,基于linux2.6.36的kernel版本:
在开启CONFIG_SMP情况下per_cpu宏的实现如下,其功能是根据cpu的值计算其内部数据偏移(per_cpu_offset(cpu)),返回var指针增加该偏移后的地址:
#define per_cpu(var, cpu) \ (*SHIFT_PERCPU_PTR(&(var), per_cpu_offset(cpu)))
其中,SHIFT_PERCPU_PTR 宏的定义如下,首先验证 __p 是否为一个 percpu 变量指针,然后使 __p 的值加 __offset 。分析如下:
/* Weird cast keeps both GCC and sparse happy. */ #define SHIFT_PERCPU_PTR(__p, __offset) ({ \ __verify_pcpu_ptr((__p)); \ //1 RELOC_HIDE((typeof(*(__p)) __kernel __force *)(__p), (__offset)); \ //2 })
//1验证指针的宏:
/* * Macro which verifies @ptr is a percpu pointer without evaluating * @ptr. This is to be used in percpu accessors to verify that the * input parameter is a percpu pointer. */ #define __verify_pcpu_ptr(ptr) do { \ const void __percpu *__vpp_verify = (typeof(ptr))NULL; \ (void)__vpp_verify; \ } while (0)
//2使指针增量的宏:
/* * This macro obfuscates arithmetic on a variable address so that gcc * shouldn't recognize the original var, and make assumptions about it. * * This is needed because the C standard makes it undefined to do * pointer arithmetic on "objects" outside their boundaries and the * gcc optimizers assume this is the case. In particular they * assume such arithmetic does not wrap. * * A miscompilation has been observed because of this on PPC. * To work around it we hide the relationship of the pointer and the object * using this macro. * * Versions of the ppc64 compiler before 4.1 had a bug where use of * RELOC_HIDE could trash r30. The bug can be worked around by changing * the inline assembly constraint from =g to =r, in this particular * case either is valid. */ #define RELOC_HIDE(ptr, off) \ ({ unsigned long __ptr; \ __asm__ ("" : "=r"(__ptr) : "0"(ptr)); \ //__ptr存放在寄存器中,ptr存放在__ptr所在寄存器中,即完成ptr到__ptr的赋值,最后将__ptr的值返回 (typeof(ptr)) (__ptr + (off)); }) //整个实现将ptr的值加上off,返回给ptr
关于per_cpu_offset的宏定义如下,per_cpu_offset(x)的值实际上是trap_block数组以x作为数组下标的数组项中结构体trap_per_cpu的__per_cpu_base的值:
/* * per_cpu_offset() is the offset that has to be added to a * percpu variable to get to the instance for a certain processor. * * Most arches use the __per_cpu_offset array for those offsets but * some arches have their own ways of determining the offset (x86_64, s390). */ #define per_cpu_offset(x) (__per_cpu_offset(x)) #define __per_cpu_offset(__cpu) \ (trap_block[(__cpu)].__per_cpu_base) struct trap_per_cpu trap_block[NR_CPUS];
Reference:
[1]http://www.ibiblio.org/gferg/ldp/GCC-Inline-Assembly-HOWTO.html#s6