Inline assembly Unable to dynamically access RISC-V CSRs

[alistair23]

With the new inline assembly it would be nice to be able to read/write from a RISC-V CSR, where the CSR value is passed in via a function argument (or some other means).

Currently there is no way to access the CSR without hard coding the address in the assembly.

const

I tried this code:

let value: usize = 0x3A0;
let r: u32;
unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = const value) }
r

Which results in this error:

error: argument 2 is required to be a constant
  --> libraries/riscv-csr/src/csr.rs:39:18
   |
39 |         unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = const value) }
   |                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

in(reg)

I have tried this code:

let value: usize = 0x3A0;
let r: u32;
unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = in(reg) value) }
r

Which results in this error:

error: operand must be a valid system register name or an integer in the range [0, 4095]
  |
note: instantiated into assembly here
 --> <inline asm>:1:11
  |
1 |     csrr a4, s6

in(csr)

and I have tried this code:

let value: usize = 0x3A0;
let r: u32;
unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = in(csr) value) }
r

Which results in this error:

error: invalid register class `csr`: unknown register class
  --> libraries/riscv-csr/src/csr.rs:39:60
   |
39 |         unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = in(csr) value) }
   |                      

Obviously all of the above are incorrect as defined in the RFC, so that is fine.

It would be great if there is either a in(csr) option or the const option is expanded to allow non-consts (maybe a num option or something).

This seems like a common usecase to have a read_csr(csr_num: usize) -> u32 function that can read a CSR value from a number passed in.

Meta

rustc --version --verbose:

rustc 1.45.0-nightly (fe10f1a49 2020-06-02)
binary: rustc
commit-hash: fe10f1a49f5ca46e57261b95f46f519523f418fe
commit-date: 2020-06-02
host: x86_64-unknown-linux-gnu
release: 1.45.0-nightly
LLVM version: 10.0

Backtrace

<backtrace>

[jonas-schievink]

The CSR is encoded as an immediate, so it must be a value statically known at compile-time. Thus, closing as expected behavior.

[alistair23]

So the plan is to never allow accessing them without hard coding the assembly?

[jonas-schievink]

Well, "hard coding the assembly" is what a compiler does. Otherwise this would require generating or modifying machine code at runtime, which is something much more complex than inline assembly (IIRC GCC does it for a GNU C extension, breaking non-executable stacks in the process).

[alistair23]

I was hoping we could have something like:

    fn read_csr(csr_num: usize) -> u32 {
        let r: u32;
        unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = csr_num) }
        r
    }

Then the calls to the function would produce assembly like this:

read_csr(0x10) which generates: csrr a0, 0x10
read_csr(0x20) which generates: csrr a0, 0x20

I see what you mean though that it would then be possible to have: read_csr(user_input * 10) which would require run-time generating of the instruction. Is it possible to force the input to be known at compile time?

Otherwise we would have to handwrite something like this:

    fn read_csr(csr_num: usize) -> u32 {
        let r: u32;
        if csr_num == 0x10 {
            unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = 0x10) }
        } else if csr_num == 0x20 {
            unsafe { asm!("csrr {rd}, {csr}", rd = out(reg) r, csr = 0x20) }
        }
        r
    }

[jonas-schievink]

I'm not sure, but I can suggest asking questions on https://users.rust-lang.org/. Maybe using a trait with an associated const works?

[lenary]

@alistair23 I've got a proposal somewhere for LLVM and Clang builtins which would support this (as long as you can get LLVM to compile down the csr identifier to a constant). The path to upstreaming them probably involves a proper proposal to sw-dev though, and agreement on the intrinsic names.