Linux arm64如何在AArch32和AArch64之间切换 [英] How Linux arm64 switch between AArch32 and AArch64

问题描述

Linux支持运行32位应用程序，只要

Linux supports running 32-bit application, as long as

1. 内核启用 CONFIG_COMPAT
2. 硬件支持AArch32

我假设32位应用程序必须在arm AArch32执行状态下运行，并且该环境具有32位应用程序和64位应用程序.

I assume that 32-bit application must run in arm AArch32 execution state and if the environment has 32-bit application and 64-bit application.

32位应用程序->臂状态为AArch32

32-bit application process -> arm state is AArch32

64位应用程序进程和内核-> arm状态为AArch64

64-bit application process and kernel -> arm state is AArch64

对吗?

如果是，
Linux如何处理AArch32和AArch64开关?
内核知道运行的进程是32位还是64位吗?

If so,
how does the Linux handle the AArch32 and AArch64 switch?
Does the kernel know the running process is 32-bit or 64-bit?

推荐答案

链接0andriy (内核开发人员)通过马丁·魏德曼解释了如何在AArch32用户空间进程和AArch64 Linux内核之间进行切换.一个>.在例外情况下完成了32-> 64模式切换；并且在异常返回时完成了64-> 32切换.

The link https://community.arm.com/developer/ip-products/processors/f/cortex-a-forum/6706/in-aarch32-state-what-is-the-mechanism-to-switch-to-aarch64-in-software posted in comments by 0andriy (kernel developer) has explanation of switching between AArch32 user-space process and AArch64 linux kernel by Martin Weidmann. The 32->64 mode switch is done at exceptions; and 64->32 switch is done at exception return.

如果您当前正在运行32位应用程序之一，并且遇到异常(例如IRQ，系统调用中的SVC，因页面错误而中止...)，则进入64位操作系统.因此是AArch32-> AArch64过渡.当操作系统执行异常时，返回到应用程序，这是AArch64-> AArch32过渡....AArch32状态下的任何异常类型都可能导致执行状态更改为AArch64....对于异常返回，反之亦然.AArch64中的异常返回可能导致执行状态更改为AArch32.
对于异常和异常返回，仅在EL发生更改的情况下，执行状态也会发生更改.从EL0到EL1是一个例外，这可能导致执行状态发生变化.但是从EL1到EL1的例外是不可能的.

If you are currently running one of the 32-bit apps and you take an exception (e.g. IRQ, SVC from a system call, abort from a page fault,....) you enter the 64-bit OS. So a AArch32 --> AArch64 transition. When the OS performs an exception return back into the app, that's an AArch64-->AArch32 transition. ... Any exception type in AArch32 state could potentially lead to Execution state changing to AArch64. ... For exceptions returns the reverse is true. An exception return in AArch64 might cause execution state to change to AArch32.
For both exceptions and exception returns, a change of Execution state can only occur if there is also a change in EL. That is an exception from EL0 to EL1 could lead to a change in Execution state. But an exception from EL1 to EL1 could not.

https://medium.com/@om上有更简单的解释.nara/aarch64-exception-levels-60d3a74280e6 在在AArch32和AArch64之间移动"

The https://community.arm.com/developer/ip-products/processors/f/cortex-a-forum/6706/in-aarch32-state-what-is-the-mechanism-to-switch-to-aarch64-in-software thread has some more details. Or there is much simpler explanation in https://medium.com/@om.nara/aarch64-exception-levels-60d3a74280e6 in "Moving between AArch32 and AArch64"

在处理异常时，如果异常级别发生变化，则执行状态可以:保持不变，或从AArch32更改为AArch64.

On taking an exception, if the Exception level changes, the Execution state can: Remain unchanged, OR Change from AArch32 to AArch64.

从异常返回时，如果异常级别更改，则执行状态可以:保持不变，或从AArch64更改为AArch32.

On returning from an exception, if the Exception level changes, the Execution state can: Remain unchanged, OR Change from AArch64 to AArch32.

相同于 https://events.static.linuxfound.org/images/stories/pdf/lcna_co2012_marinas.pdf 演示文稿(幻灯片5)或在 https://developer.arm.com/architectures/learn-the-architecture/exception-model/execution-and-security-states 或在 https://www.realworldtech.com/arm64/2/:

Same in https://events.static.linuxfound.org/images/stories/pdf/lcna_co2012_marinas.pdf presentation (slide 5) or in https://developer.arm.com/architectures/learn-the-architecture/exception-model/execution-and-security-states or in https://www.realworldtech.com/arm64/2/:

AArch64异常模型

AArch64 Exception Model

• 特权级别:EL3-最高，EL0-最低

• 通过异常过渡到更高级别

• Privilege levels: EL3 – highest, EL0 – lowest

• Transition to higher levels via exceptions

在较低级别中，寄存器宽度不能更大

Register width cannot be higher in lower levels

• 例如没有64位EL0和32位EL1

• 无法实现AArch32/AArch64互通

现在提出您的问题:

Linux如何处理AArch32和AArch64开关?

how does the Linux handle the AArch32 and AArch64 switch?

通过对具有不同PSTATE值的EL0/EL1开关使用异常处理(和返回)的硬件功能.

By using hardware capability of exception handling (and return) with EL0/EL1 switch with different PSTATE values.

内核知道运行的进程是32位还是64位吗?

Does the kernel know the running process is 32-bit or 64-bit?

是的，在64位内核(compat syscalls)上有32位进程(任务")的签入内核:

Yes, there is the check in kernel for 32-bit processes ("tasks") on 64-bit kernels (compat syscalls): arch/arm64/kernel/syscall.c

static long do_ni_syscall(struct pt_regs *regs, int scno)
{
#ifdef CONFIG_COMPAT
    long ret;
    if (is_compat_task()) {
        ret = compat_arm_syscall(regs, scno);
        if (ret != -ENOSYS)
            return ret;
    }
#endif

    return sys_ni_syscall();
}

测试在 include/asm/compat.h 和 arch/arm64/include/asm/thread_info.h as

#define TIF_32BIT       22  /* 32bit process */
static inline int is_compat_task(void)
{
    return test_thread_flag(TIF_32BIT);
}

TIF_32BIT由fs/compat_binfmt_elf.c在32位elf加载中通过几个个性宏设置:

TIF_32BIT is set by in fs/compat_binfmt_elf.c on 32-bit elf loading with several personality macro:

https://elixir.bootlin.com/linux/v4.19.107/source/arch/arm64/include/asm/elf.h#L208

/*
 * Unlike the native SET_PERSONALITY macro, the compat version maintains
 * READ_IMPLIES_EXEC across an execve() since this is the behaviour on
 * arch/arm/.
 */
#define COMPAT_SET_PERSONALITY(ex)                  \
({                                  \
    set_thread_flag(TIF_32BIT);                 \
 })

https://elixir.bootlin.com/linux/v4.19.107/source/fs/compat_binfmt_elf.c#L104

 #define    SET_PERSONALITY     COMPAT_SET_PERSONALITY

https://elixir.bootlin.com/linux/v4.19.107/source/fs/binfmt_elf.c#L690

#define SET_PERSONALITY2(ex, state) \
    SET_PERSONALITY(ex)
 static int load_elf_binary(struct linux_binprm *bprm)
    SET_PERSONALITY2(loc->elf_ex, &arch_state);

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