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# 教程 09 - 特权级别
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## tl;dr
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- 在早期引导代码中，我们从`Hypervisor`特权级别（AArch64中的`EL2`）过渡到`Kernel` （`EL1`）特权级别。
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## 目录
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- [介绍](#介绍)
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- [本教程的范围](#本教程的范围)
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- [在入口点检查EL2](#在入口点检查EL2)
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- [过渡准备](#过渡准备)
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- [从未发生的异常中返回](#从未发生的异常中返回)
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- [测试](#测试)
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- [相比之前的变化（diff）](#相比之前的变化（diff）)
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## 介绍
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应用级别的CPU具有所谓的`privilege levels`，它们具有不同的目的：
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| Typically used for | AArch64 | RISC-V | x86 |
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| ------------- | ------------- | ------------- | ------------- |
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| Userspace applications | EL0 | U/VU | Ring 3 |
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| OS Kernel | EL1 | S/VS | Ring 0 |
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| Hypervisor | EL2 | HS | Ring -1 |
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| Low-Level Firmware | EL3 | M | |
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在AArch64中，`EL`代表`Exception Level`（异常级别）。如果您想获取有关其他体系结构的更多信息，请查看以下链接：
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- [x86 privilege rings](https://en.wikipedia.org/wiki/Protection_ring).
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- [RISC-V privilege modes](https://content.riscv.org/wp-content/uploads/2017/12/Tue0942-riscv-hypervisor-waterman.pdf).
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在继续之前，我强烈建议您先浏览一下[Programmer’s Guide for ARMv8-A]`的第3章`。它提供了关于该主题的简明概述。
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[Programmer’s Guide for ARMv8-A]: http://infocenter.arm.com/help/topic/com.arm.doc.den0024a/DEN0024A_v8_architecture_PG.pdf
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## 本教程的范围
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默认情况下，树莓派将始终在`EL2`中开始执行。由于我们正在编写一个传统的`Kernel`，我们需要过渡到更合适的`EL1`。
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## 在入口点检查EL2
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首先，我们需要确保我们实际上是在`EL2`中执行，然后才能调用相应的代码过渡到`EL1`。
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因此，我们在`boot.s`的顶部添加了一个新的检查，如果CPU核心不在`EL2`中，则将其停止。
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```
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// Only proceed if the core executes in EL2. Park it otherwise.
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mrs x0, CurrentEL
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cmp x0, {CONST_CURRENTEL_EL2}
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b.ne .L_parking_loop
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```
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接下来，在`boot.rs`中继续准备从`EL2`到`EL1`的过渡，通过调用`prepare_el2_to_el1_transition()`函数。
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```rust
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#[no_mangle]
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pub unsafe extern "C" fn _start_rust(phys_boot_core_stack_end_exclusive_addr: u64) -> ! {
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prepare_el2_to_el1_transition(phys_boot_core_stack_end_exclusive_addr);
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// Use `eret` to "return" to EL1. This results in execution of kernel_init() in EL1.
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asm::eret()
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}
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```
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## 过渡准备
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由于`EL2`比`EL1`更具特权，它可以控制各种处理器功能，并允许或禁止`EL1`代码使用它们。
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其中一个例子是访问计时器和计数器寄存器。我们已经在[tutorial 07](../07_timestamps/)中使用了它们，所以当然我们希望保留它们。
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因此，我们在[Counter-timer Hypervisor Control register]中设置相应的标志，并将虚拟偏移量设置为零，以获取真实的物理值。
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[Counter-timer Hypervisor Control register]: https://docs.rs/aarch64-cpu/9.0.0/src/aarch64_cpu/registers/cnthctl_el2.rs.html
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```rust
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// Enable timer counter registers for EL1.
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CNTHCTL_EL2.write(CNTHCTL_EL2::EL1PCEN::SET + CNTHCTL_EL2::EL1PCTEN::SET);
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// No offset for reading the counters.
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CNTVOFF_EL2.set(0);
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```
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接下来，我们配置[Hypervisor Configuration Register]，使`EL1`在`AArch64`模式下运行，而不是在`AArch32`模式下运行，这也是可能的。
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[Hypervisor Configuration Register]: https://docs.rs/aarch64-cpu/9.0.0/src/aarch64_cpu/registers/hcr_el2.rs.html
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```rust
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// Set EL1 execution state to AArch64.
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HCR_EL2.write(HCR_EL2::RW::EL1IsAarch64);
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```
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## 从未发生的异常中返回
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实际上，从较高的EL过渡到较低的EL只有一种方式，即通过执行[ERET]指令。
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[ERET]: https://docs.rs/aarch64-cpu/9.0.0/src/aarch64_cpu/asm.rs.html#92-101
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在这个指令中，它将会将[Saved Program Status Register - EL2]的内容复制到`Current Program Status Register - EL1`，并跳转到存储在[Exception Link Register - EL2]。
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这基本上是在发生异常时所发生的相反过程。您将在即将发布的教程中了解更多相关内容。
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[Saved Program Status Register - EL2]: https://docs.rs/aarch64-cpu/9.0.0/src/aarch64_cpu/registers/spsr_el2.rs.html
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[Exception Link Register - EL2]: https://docs.rs/aarch64-cpu/9.0.0/src/aarch64_cpu/registers/elr_el2.rs.html
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```rust
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// Set up a simulated exception return.
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//
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// First, fake a saved program status where all interrupts were masked and SP_EL1 was used as a
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// stack pointer.
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SPSR_EL2.write(
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SPSR_EL2::D::Masked
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+ SPSR_EL2::A::Masked
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+ SPSR_EL2::I::Masked
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+ SPSR_EL2::F::Masked
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+ SPSR_EL2::M::EL1h,
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);
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// Second, let the link register point to kernel_init().
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ELR_EL2.set(crate::kernel_init as *const () as u64);
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// Set up SP_EL1 (stack pointer), which will be used by EL1 once we "return" to it. Since there
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// are no plans to ever return to EL2, just re-use the same stack.
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SP_EL1.set(phys_boot_core_stack_end_exclusive_addr);
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```
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正如您所看到的，我们将`ELR_EL2`的值设置为之前直接从入口点调用的`kernel_init()`函数的地址。最后，我们设置了`SP_EL1`的堆栈指针。
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您可能已经注意到，堆栈的地址作为函数参数进行了传递。正如您可能记得的，在`boot.s`的`_start()`函数中，
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我们已经为`EL2`设置了堆栈。由于没有计划返回到`EL2`，我们可以直接重用相同的堆栈作为`EL1`的堆栈，
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因此使用函数参数将其地址传递。
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最后，在`_start_rust()`函数中调用了`ERET`指令。
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```rust
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#[no_mangle]
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pub unsafe extern "C" fn _start_rust(phys_boot_core_stack_end_exclusive_addr: u64) -> ! {
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prepare_el2_to_el1_transition(phys_boot_core_stack_end_exclusive_addr);
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// Use `eret` to "return" to EL1. This results in execution of kernel_init() in EL1.
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asm::eret()
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}
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```
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## 测试
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在`main.rs`中，我们打印`current privilege level`，并额外检查`SPSR_EL2`中的掩码位是否传递到了`EL1`：
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```console
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$ make chainboot
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[...]
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Minipush 1.0
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[MP] ⏳ Waiting for /dev/ttyUSB0
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[MP] ✅ Serial connected
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[MP] 🔌 Please power the target now
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__ __ _ _ _ _
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| \/ (_)_ _ (_) | ___ __ _ __| |
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| |\/| | | ' \| | |__/ _ \/ _` / _` |
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|_| |_|_|_||_|_|____\___/\__,_\__,_|
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Raspberry Pi 3
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[ML] Requesting binary
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[MP] ⏩ Pushing 14 KiB =========================================🦀 100% 0 KiB/s Time: 00:00:00
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[ML] Loaded! Executing the payload now
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[ 0.162546] mingo version 0.9.0
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[ 0.162745] Booting on: Raspberry Pi 3
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[ 0.163201] Current privilege level: EL1
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[ 0.163677] Exception handling state:
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[ 0.164122] Debug: Masked
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[ 0.164511] SError: Masked
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[ 0.164901] IRQ: Masked
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[ 0.165291] FIQ: Masked
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[ 0.165681] Architectural timer resolution: 52 ns
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[ 0.166255] Drivers loaded:
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[ 0.166592] 1. BCM PL011 UART
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[ 0.167014] 2. BCM GPIO
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[ 0.167371] Timer test, spinning for 1 second
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[ 1.167904] Echoing input now
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```
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## 相比之前的变化（diff）
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请检查[英文版本](README.md#diff-to-previous)，这是最新的。
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James Zow
7 months ago
committed by
GitHub