Linux kernel 中的per_cpu宏

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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));\        //1RELOC_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

[2]http://www.yubo.org/xen/code.php?file=linux-2.6.18-xen.hg/include/linux/compiler-gcc.h&begin=20&end=23&lang=CPP&num=1&version=3.4&title=RELOC_HIDE()