rust-raspberrypi-OS-tutorials/10_privilege_level

Tutorial 10 - Privilege Level

tl;dr

In early boot code, we transition from the Hypervisor privilege level (EL2 in AArch64) to the Kernel (EL1) privilege level.

Introduction

Application-grade CPUs have so-called privilege levels, which have different purposes:

Typically used for	AArch64	RISC-V	x86
Userspace applications	EL0	U/VU	Ring 3
OS Kernel	EL1	S/VS	Ring 0
Hypervisor	EL2	HS	Ring -1
Low-Level Firmware	EL3	M

EL in AArch64 stands for Exception Level. If you want more information regarding the other architectures, please have a look at the following links:

• x86 privilege rings.
• RISC-V privilege modes.

At this point, I strongly recommend that you glimpse over Chapter 3 of the Programmer’s Guide for ARMv8-A before you continue. It gives a concise overview about the topic.

Scope of this tutorial

If you set up your SD Card exactly like mentioned in tutorial 06, the Rpi will always start executing in EL2. Since we are writing a traditional Kernel, we have to transition into the more appropriate EL1.

Checking for EL2 in the entrypoint

First of all, we need to ensure that we actually execute in EL2 before we can call respective code to transition to EL1:

pub unsafe extern "C" fn _start() -> ! {
    const CORE_MASK: u64 = 0x3;

    // Expect the boot core to start in EL2.
    if (bsp::BOOT_CORE_ID == MPIDR_EL1.get() & CORE_MASK)
        && (CurrentEL.get() == CurrentEL::EL::EL2.value)
    {
        el2_to_el1_transition()
    } else {
        // If not core0, infinitely wait for events.
        wait_forever()
    }
}

If this is the case, we continue with preparing the EL2 -> EL1 transition in el2_to_el1_transition().

Transition preparation

Since EL2 is more privileged than EL1, it has control over various processor features and can allow or disallow EL1 code to use them. One such example is access to timer and counter registers. We are already using them since tutorial 08, so of course we want to keep them. Therefore we set the respective flags in the Counter-timer Hypervisor Control register and additionally set the virtual offset to zero so that we get the real physical value everytime:

// Enable timer counter registers for EL1.
CNTHCTL_EL2.write(CNTHCTL_EL2::EL1PCEN::SET + CNTHCTL_EL2::EL1PCTEN::SET);

// No offset for reading the counters.
CNTVOFF_EL2.set(0);

Next, we configure the Hypervisor Configuration Register such that EL1 should actually run in AArch64 mode, and not in AArch32, which would also be possible.

// Set EL1 execution state to AArch64.
HCR_EL2.write(HCR_EL2::RW::EL1IsAarch64);

Returning from an exception that never happened

There is actually only one way to transition from a higher EL to a lower EL, which is by way of executing the ERET instruction.

This instruction will copy the contents of the Saved Program Status Register - EL2 to Current Program Status Register - EL1 and jump to the instruction address that is stored in the Exception Link Register - EL2.

This is basically the reverse of what is happening when an exception is taken. You'll learn about it in an upcoming tutorial.

// Set up a simulated exception return.
//
// First, fake a saved program status, where all interrupts were masked and SP_EL1 was used as a
// stack pointer.
SPSR_EL2.write(
    SPSR_EL2::D::Masked
        + SPSR_EL2::A::Masked
        + SPSR_EL2::I::Masked
        + SPSR_EL2::F::Masked
        + SPSR_EL2::M::EL1h,
);

// Second, let the link register point to init().
ELR_EL2.set(crate::runtime_init::init as *const () as u64);

As you can see, we are populating ELR_EL2 with the address of the init() function that we earlier used to call directly from the entrypoint.

Finally, we set the stack pointer for SP_EL1 and call ERET:

// Set up SP_EL1 (stack pointer), which will be used by EL1 once we "return" to it.
SP_EL1.set(bsp::BOOT_CORE_STACK_START);

// Use `eret` to "return" to EL1. This will result in execution of `reset()` in EL1.
asm::eret()

Are we stackless?

We just wrote a big inline rust function, el2_to_el1_transition(), that is executed in a context where we do not have a stack yet. We should double-check the generated machine code:

make objdump
[...]
Disassembly of section .text:

0000000000080000 _start:
   80000: a8 00 38 d5                   mrs     x8, MPIDR_EL1
   80004: 1f 05 40 f2                   tst     x8, #0x3
   80008: 81 00 00 54                   b.ne    #0x10 <_start+0x18>
   8000c: 48 42 38 d5                   mrs     x8, CurrentEL
   80010: 1f 21 00 71                   cmp     w8, #0x8
   80014: 60 00 00 54                   b.eq    #0xc <_start+0x20>
   80018: 5f 20 03 d5                   wfe
   8001c: ff ff ff 17                   b       #-0x4 <_start+0x18>
   80020: e8 03 1f aa                   mov     x8, xzr
   80024: 69 00 80 52                   mov     w9, #0x3
   80028: 09 e1 1c d5                   msr     CNTHCTL_EL2, x9
   8002c: 68 e0 1c d5                   msr     CNTVOFF_EL2, x8
   80030: 08 00 00 90                   adrp    x8, #0x0
   80034: 0a 00 b0 52                   mov     w10, #-0x80000000
   80038: ab 78 80 52                   mov     w11, #0x3c5
   8003c: 0c 01 a0 52                   mov     w12, #0x80000
   80040: 0a 11 1c d5                   msr     HCR_EL2, x10
   80044: 0b 40 1c d5                   msr     SPSR_EL2, x11
   80048: 08 91 2f 91                   add     x8, x8, #0xbe4
   8004c: 28 40 1c d5                   msr     ELR_EL2, x8
   80050: 0c 41 1c d5                   msr     SP_EL1, x12
   80054: e0 03 9f d6                   eret

Looks good! Thanks zero-overhead abstractions in the cortex-a crate! 😍

Testing

In main.rs, we additionally inspect if the mask bits in SPSR_EL2 made it to EL1 as well:

make chainbot
[...]
### Listening on /dev/ttyUSB0
 __  __ _      _ _                 _
|  \/  (_)_ _ (_) |   ___  __ _ __| |
| |\/| | | ' \| | |__/ _ \/ _` / _` |
|_|  |_|_|_||_|_|____\___/\__,_\__,_|

           Raspberry Pi 3

[ML] Requesting binary
### sending kernel kernel8.img [16480 byte]
### finished sending
[ML] Loaded! Executing the payload now

[    1.459973] Booting on: Raspberry Pi 3
[    1.462256] Current privilege level: EL1
[    1.466163] Exception handling state:
[    1.469810]       Debug:  Masked
[    1.473023]       SError: Masked
[    1.476235]       IRQ:    Masked
[    1.479447]       FIQ:    Masked
[    1.482661] Architectural timer resolution: 52 ns
[    1.487349] Drivers loaded:
[    1.490127]       1. GPIO
[    1.492731]       2. PL011Uart
[    1.495770] Timer test, spinning for 1 second
[    2.500114] Echoing input now

Diff to previous

diff -uNr 09_hw_debug_JTAG/src/arch/aarch64/exception.rs 10_privilege_level/src/arch/aarch64/exception.rs
--- 09_hw_debug_JTAG/src/arch/aarch64/exception.rs
+++ 10_privilege_level/src/arch/aarch64/exception.rs
@@ -0,0 +1,48 @@
+// SPDX-License-Identifier: MIT OR Apache-2.0
+//
+// Copyright (c) 2018-2019 Andre Richter <andre.o.richter@gmail.com>
+
+//! Exception handling.
+
+use cortex_a::regs::*;
+
+//--------------------------------------------------------------------------------------------------
+// Arch-public
+//--------------------------------------------------------------------------------------------------
+
+pub trait DaifField {
+    fn daif_field() -> register::Field<u32, DAIF::Register>;
+}
+
+pub struct Debug;
+pub struct SError;
+pub struct IRQ;
+pub struct FIQ;
+
+impl DaifField for Debug {
+    fn daif_field() -> register::Field<u32, DAIF::Register> {
+        DAIF::D
+    }
+}
+
+impl DaifField for SError {
+    fn daif_field() -> register::Field<u32, DAIF::Register> {
+        DAIF::A
+    }
+}
+
+impl DaifField for IRQ {
+    fn daif_field() -> register::Field<u32, DAIF::Register> {
+        DAIF::I
+    }
+}
+
+impl DaifField for FIQ {
+    fn daif_field() -> register::Field<u32, DAIF::Register> {
+        DAIF::F
+    }
+}
+
+pub fn is_masked<T: DaifField>() -> bool {
+    DAIF.is_set(T::daif_field())
+}

diff -uNr 09_hw_debug_JTAG/src/arch/aarch64.rs 10_privilege_level/src/arch/aarch64.rs
--- 09_hw_debug_JTAG/src/arch/aarch64.rs
+++ 10_privilege_level/src/arch/aarch64.rs
@@ -4,6 +4,7 @@

 //! AArch64.

+mod exception;
 pub mod sync;
 mod time;

@@ -21,15 +22,56 @@
 pub unsafe extern "C" fn _start() -> ! {
     const CORE_MASK: u64 = 0x3;

-    if bsp::BOOT_CORE_ID == MPIDR_EL1.get() & CORE_MASK {
-        SP.set(bsp::BOOT_CORE_STACK_START);
-        crate::runtime_init::init()
+    // Expect the boot core to start in EL2.
+    if (bsp::BOOT_CORE_ID == MPIDR_EL1.get() & CORE_MASK)
+        && (CurrentEL.get() == CurrentEL::EL::EL2.value)
+    {
+        el2_to_el1_transition()
     } else {
         // If not core0, infinitely wait for events.
         wait_forever()
     }
 }

+/// Transition from EL2 to EL1.
+///
+/// # Safety
+///
+/// - The HW state of EL1 must be prepared in a sound way.
+/// - Exception return from EL2 must must continue execution in EL1 with ´runtime_init::init()`.
+#[inline(always)]
+unsafe fn el2_to_el1_transition() -> ! {
+    // Enable timer counter registers for EL1.
+    CNTHCTL_EL2.write(CNTHCTL_EL2::EL1PCEN::SET + CNTHCTL_EL2::EL1PCTEN::SET);
+
+    // No offset for reading the counters.
+    CNTVOFF_EL2.set(0);
+
+    // Set EL1 execution state to AArch64.
+    HCR_EL2.write(HCR_EL2::RW::EL1IsAarch64);
+
+    // Set up a simulated exception return.
+    //
+    // First, fake a saved program status, where all interrupts were masked and SP_EL1 was used as a
+    // stack pointer.
+    SPSR_EL2.write(
+        SPSR_EL2::D::Masked
+            + SPSR_EL2::A::Masked
+            + SPSR_EL2::I::Masked
+            + SPSR_EL2::F::Masked
+            + SPSR_EL2::M::EL1h,
+    );
+
+    // Second, let the link register point to init().
+    ELR_EL2.set(crate::runtime_init::init as *const () as u64);
+
+    // Set up SP_EL1 (stack pointer), which will be used by EL1 once we "return" to it.
+    SP_EL1.set(bsp::BOOT_CORE_STACK_START);
+
+    // Use `eret` to "return" to EL1. This will result in execution of `reset()` in EL1.
+    asm::eret()
+}
+
 //--------------------------------------------------------------------------------------------------
 // Global instances
 //--------------------------------------------------------------------------------------------------
@@ -61,3 +103,36 @@
         asm::wfe()
     }
 }
+
+/// Information about the HW state.
+pub mod state {
+    use cortex_a::regs::*;
+
+    /// The processing element's current privilege level.
+    pub fn current_privilege_level() -> &'static str {
+        let el = CurrentEL.read_as_enum(CurrentEL::EL);
+        match el {
+            Some(CurrentEL::EL::Value::EL2) => "EL2",
+            Some(CurrentEL::EL::Value::EL1) => "EL1",
+            _ => "Unknown",
+        }
+    }
+
+    #[rustfmt::skip]
+    pub fn print_exception_state() {
+        use super::{
+            exception,
+            exception::{Debug, SError, FIQ, IRQ},
+        };
+        use crate::info;
+
+        let to_mask_str = |x: bool| -> &'static str {
+            if x { "Masked" } else { "Unmasked" }
+        };
+
+        info!("      Debug:  {}", to_mask_str(exception::is_masked::<Debug>()));
+        info!("      SError: {}", to_mask_str(exception::is_masked::<SError>()));
+        info!("      IRQ:    {}", to_mask_str(exception::is_masked::<IRQ>()));
+        info!("      FIQ:    {}", to_mask_str(exception::is_masked::<FIQ>()));
+    }
+}

diff -uNr 09_hw_debug_JTAG/src/main.rs 10_privilege_level/src/main.rs
--- 09_hw_debug_JTAG/src/main.rs
+++ 10_privilege_level/src/main.rs
@@ -63,9 +63,17 @@
 /// The main function running after the early init.
 fn kernel_main() -> ! {
     use core::time::Duration;
-    use interface::time::Timer;
+    use interface::{console::All, time::Timer};

     info!("Booting on: {}", bsp::board_name());
+
+    info!(
+        "Current privilege level: {}",
+        arch::state::current_privilege_level()
+    );
+    info!("Exception handling state:");
+    arch::state::print_exception_state();
+
     info!(
         "Architectural timer resolution: {} ns",
         arch::timer().resolution().as_nanos()
@@ -76,11 +84,12 @@
         info!("      {}. {}", i + 1, driver.compatible());
     }

-    // Test a failing timer case.
-    arch::timer().spin_for(Duration::from_nanos(1));
+    info!("Timer test, spinning for 1 second");
+    arch::timer().spin_for(Duration::from_secs(1));

+    info!("Echoing input now");
     loop {
-        info!("Spinning for 1 second");
-        arch::timer().spin_for(Duration::from_secs(1));
+        let c = bsp::console().read_char();
+        bsp::console().write_char(c);
     }
 }