Rustc does not recognize ttbr0_el2 as a valid armv8 system register

[iankronquist]

Rustc does not recognize ttbr0_el2 as a valid armv8 system register. ttbr0_el2 is used by hypervisors to configure page tables for the lower half of memory, typically used for userspace.

#![no_std]
use core::arch::asm;

static PT: [u64; 512] = [0; 512];
pub fn main() {
    unsafe {
        asm!("msr {pt}, ttbr0_el2", pt = in(reg) &PT as *const _ );
    }
}

This fails to compile with the error:

error: expected writable system register or pstate
 --> <source>:8:15
  |
8 |         asm!("msr {pt}, ttbr0_el2", pt = in(reg) &PT as *const _ );
  |               ^
  |
note: instantiated into assembly here
 --> <inline asm>:1:6
  |
1 |     msr x8, ttbr0_el2
  |         ^

error: aborting due to previous error

Compiler returned: 1

Here is a live example on godbolt: https://godbolt.org/z/3jsGEd59a

My rustc version is:

$ rustc --version
rustc 1.63.0-nightly (e71440575 2022-06-02)

[asquared31415]

That is an assembler error, nothing to do directly with Rust.

I think your arguments are backwards. To move from PT into that register, it would me msr ttbr0_el2, {pt}.

Edit: upon further inspection, even with that fix, LLVM doesn't seem to recognize the ttbr0_el2 register specifically (1 and 3 work)

[x1tan]

Running the latest nightly build (rustc 1.64.0-nightly (830880640 2022-06-28)) still produces the same error. Possible (temporary) workaround: creating a custom aarch64 target and adding +v8.1a to its features.

[adamgemmell]

As the above issue says it's locked behind the el2vmsa LLVM feature, which Rust doesn't directly expose. You can turn it on with the unstable v8.1a feature for now: https://godbolt.org/z/6vcd3aYEM

[iankronquist]

Awesome thanks for the update! I believe the bug can be closed now.Ian KronquistOn Sep 23, 2022, at 03:38, Adam Gemmell @.***> wrote: As the above issue says it's locked behind the el2vmsa LLVM feature, which Rust doesn't directly expose. You can turn it on with the unstable v8.1a feature for now: https://godbolt.org/z/6vcd3aYEM

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[oToToT]

I believe the code now compiles in the latest stable (1.67.0)