试图从linux内核中理解一个内联臂组件

Question

我试图理解这个内联汇编的内容，它是从C DEFINE宏生成的：

#define DEFINE(sym, val) asm volatile("\n->" #sym " %0 " #val : : "i" (val))

#define offsetof(TYPE, MEMBER)  ((size_t)&((TYPE *)0)->MEMBER)

它用于在smc内核fork中进行AArch64 linux-imx调用的一些魔法。上面的C宏用于定义值的位置是here：

// arch/arm64/kernel/asm-offsets.c

int main(void)
{
    (...)
      DEFINE(ARM_SMCCC_RES_X0_OFFS,     offsetof(struct arm_smccc_res, a0));
      DEFINE(ARM_SMCCC_RES_X2_OFFS,     offsetof(struct arm_smccc_res, a2));
      DEFINE(ARM_SMCCC_QUIRK_ID_OFFS,   offsetof(struct arm_smccc_quirk, id));
      DEFINE(ARM_SMCCC_QUIRK_STATE_OFFS,    offsetof(struct arm_smccc_quirk, state));
    (...)
}

后来，这些定义用于汇编宏available here（或者至少是它的样子）：

// arch/arm64/kernel/smccc-call.S

    .macro SMCCC instr
    .cfi_startproc
    \instr  #0
    ldr x4, [sp]
    stp x0, x1, [x4, #ARM_SMCCC_RES_X0_OFFS]
    stp x2, x3, [x4, #ARM_SMCCC_RES_X2_OFFS]
    ldr x4, [sp, #8]
    cbz x4, 1f /* no quirk structure */
    ldr x9, [x4, #ARM_SMCCC_QUIRK_ID_OFFS]
    cmp x9, #ARM_SMCCC_QUIRK_QCOM_A6
    b.ne    1f
    str x6, [x4, ARM_SMCCC_QUIRK_STATE_OFFS]
1:  ret
    .cfi_endproc
    .endm

ENTRY(__arm_smccc_smc)
    SMCCC   smc
ENDPROC(__arm_smccc_smc)

最终，它在实践中使用，例如在gpc-psci driver。

我试图将代码隔离到一个单独的裸机AArch64程序中，以查看宏如何扩展并在实践中工作，但我得到的只是编译错误。

// main.c
// the struct arm_smccc_res and others are inserted here to satisfy the compiler

int
main()
{
        DEFINE(ARM_SMCCC_RES_X0_OFFS,      offsetof(struct arm_smccc_res, a0));
        DEFINE(ARM_SMCCC_RES_X2_OFFS,      offsetof(struct arm_smccc_res, a2));
        DEFINE(ARM_SMCCC_QUIRK_ID_OFFS,    offsetof(struct arm_smccc_quirk, id));
        DEFINE(ARM_SMCCC_QUIRK_STATE_OFFS, offsetof(struct arm_smccc_quirk, state));
        return 0;
}

// gcc output

/tmp/cccqaaY3.s: Assembler messages:
/tmp/cccqaaY3.s:459: Error: junk at end of line, first unrecognized character is `-'
/tmp/cccqaaY3.s:464: Error: junk at end of line, first unrecognized character is `-'
/tmp/cccqaaY3.s:469: Error: junk at end of line, first unrecognized character is `-'
/tmp/cccqaaY3.s:474: Error: junk at end of line, first unrecognized character is `-'

offsetof宏是不言自明的，但我无法理解在->宏中使用字符串化和DEFINE。

有什么想法扩展到，或者如何成功编译它？

Answer 1

您会收到错误，因为该文件不应编译为对象。它仅用于生成程序集输出，然后处理它以创建标头asm_offsets.h。此标题随后包含在其他地方。你可以在include/generated找到它。创建它的规则在顶级KBuild：

# Kbuild for top-level directory of the kernel
# This file takes care of the following:
# 1) Generate bounds.h
# 2) Generate timeconst.h
# 3) Generate asm-offsets.h (may need bounds.h and timeconst.h)
# 4) Check for missing system calls
# 5) Generate constants.py (may need bounds.h)

# Default sed regexp - multiline due to syntax constraints
define sed-y
        "/^->/{s:->#\(.*\):/* \1 */:; \
        s:^->\([^ ]*\) [\$$#]*\([-0-9]*\) \(.*\):#define \1 \2 /* \3 */:; \
        s:^->\([^ ]*\) [\$$#]*\([^ ]*\) \(.*\):#define \1 \2 /* \3 */:; \
        s:->::; p;}"
endef

您可以看到它使用sed将程序集输出转换为标题。临时汇编文件可以在arch/<arch>/kernel/asm-offsets.s中找到。示例行看起来像：

->pt_regs_bx $40 offsetof(struct pt_regs, bx)   #
->pt_regs_cx $88 offsetof(struct pt_regs, cx)   #
->pt_regs_dx $96 offsetof(struct pt_regs, dx)   #

请注意，这不是有效的汇编语法，但是编译器并不关心，它会在执行参数替换后无意识地发出在asm块中放入的任何内容。然后sed命令转换它们，使标题中的匹配行看起来像：

#define pt_regs_bx 40 /* offsetof(struct pt_regs, bx)   # */
#define pt_regs_cx 88 /* offsetof(struct pt_regs, cx)   # */
#define pt_regs_dx 96 /* offsetof(struct pt_regs, dx)   # */