试图从 linux 内核中理解一段内联 arm 程序集

Question

我想了解这段内联汇编的作用，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)

它被用于在 linux-imx kernel fork. The place where the above C macros are used to define the values is here:

中执行 AArch64 smc 调用的一些魔法

// 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，available here（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 gpc-psci driver.

我试图将代码隔离到一个单独的裸机 AArch64 程序中，以查看宏是如何扩展和实际工作的，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 DEFINE 宏中字符串化和 -> 的使用。

知道它扩展到什么，或者如何成功编译它吗？

Answer 1

您收到错误是因为该文件不应编译为 object。它仅用于生成程序集输出，然后对其进行处理以创建 header asm_offsets.h。这个 header 是然后包含在其他地方的内容。您可以在 include/generated 中找到它。创建它的规则在顶层 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:->#\(.*\):/*  */:; \
        s:^->\([^ ]*\) [$$#]*\([-0-9]*\) \(.*\):#define   /*  */:; \
        s:^->\([^ ]*\) [$$#]*\([^ ]*\) \(.*\):#define   /*  */:; \
        s:->::; p;}"
endef

您可以看到它使用 sed 将程序集输出转换为 header。可以在 arch/<arch>/kernel/asm-offsets.s 中找到临时程序集文件。示例行如下所示：

->pt_regs_bx  offsetof(struct pt_regs, bx)   #
->pt_regs_cx  offsetof(struct pt_regs, cx)   #
->pt_regs_dx  offsetof(struct pt_regs, dx)   #

请注意，这不是有效的汇编语法，但编译器并不关心，它会在执行参数替换后无意识地发出您在 asm 块中输入的任何内容。 sed 命令然后转换它们，使 header 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)   # */

试图从 linux 内核中理解一段内联 arm 程序集

Trying to understand a piece of inline arm assembly from the linux kernel

c

assembly

gcc

gnu-assembler

arm64