试图理解 Linux 内核中的一段内联臂汇编

我试图了解这段内联汇编的作用，它是从 C 生成的DEFINE macro:

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

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

它被用于执行 AArch64 的一些魔法中smc调用在linux-imx内核分叉。上述C宏用于定义值的地方是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，这些定义被用在汇编宏中，可以在这里找到（或者至少看起来是这样）：

// 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)

最终在实践中使用，例如在里面gpc-psci 驱动程序.

我尝试将代码隔离到一个单独的裸机 AArch64 程序中，以查看宏在实践中如何扩展和工作，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 `-'

The offsetof宏是不言自明的，但我无法理解字符串化和-> in the DEFINE macro.

有什么想法可以扩展到什么，或者如何成功完成这个？

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，然后对其进行处理以创建标头asm_offsets.h。该标头是随后包含在其他地方的标头。您可以在以下位置找到它：include/generated。创建它的规则位于顶层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

你可以看到它使用sed将程序集输出转换为标头。临时程序集文件可以在以下位置找到arch/<arch>/kernel/asm-offsets.s。示例行如下所示：

->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，但编译器不在乎，它会无意识地发出您输入的任何内容asm进行参数替换后会阻塞。这sed然后命令对它们进行转换，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)   # */