Add loongarch64 support

[k0tran]

PR implementing loongarch64 support

Tested on loongarch64 rust 1.80.0
Builds with debug and release flags
Passes all the tests with debug and release flags

Notes to maintainer:

• loongarch32 is not supported by rust
• loongarch has 7-argument syscall. I was able to come up with some implementation but it's rather ugly in my opinion
• no clobber_abi because it clobbers more than we need

Might be useful:

• LoonArch64 ELF ABI https://loongson.github.io/LoongArch-Documentation/LoongArch-ELF-ABI-EN.html
• GLibc syscall implementation (Sysiphus repo) https://git.altlinux.org/gears/g/glibc.git?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/loongarch/sysdep.h;h=e11cd865ce5d5960e637c0c00017bb9c38874595#l131

[jasonwhite]

Awesome! I'm very skeptical that we actually need the 7 argument syscall at all. I did a little searching through the Linux kernel and didn't find a single syscall that takes 7 arguments (for any architecture). Am I missing something? I know that man syscall(2) says this architecture can have 7 arguments, but I don't see that actually happening in practice (in the Linux kernel). Is this a relic of old kernel versions?

I'd like to keep SyscallArgs at 6 arguments, since this is the common case. If I'm wrong about there not being a real life 7 argument syscall, then I would only expose it via a direct call to syscall7.

Also, it looks like loongarch64-unknown-linux-gnu is supported by cross-rs. Can you try to enable it in .github/workflows/ci.yml?

[jasonwhite]

NOTE: The CI failures for thumb mode are unrelated to this PR. It looks like a recent Rust version broke this.

[k0tran]

I found that for now there is no macro for defining 7-arg syscall
https://github.com/torvalds/linux/blob/1fb918967b56df3262ee984175816f0acb310501/include/linux/syscalls.h#L227

Will redo and force loongarch to not using syscall7

[jasonwhite]

Looks like the loongarch64 CI linker failure is an issue with cross-rs. I suspect this is just because the CI for this repo is using an outdated version of cross-rs as I see there have been some recent fixes for loongarch64.

syscalls-483deb5c79caa4ef.00d8fg4tdgpxcva2mmxyzpx89.rcgu.o: Relocations in generic ELF (EM: 258)
syscalls-483deb5c79caa4ef.00d8fg4tdgpxcva2mmxyzpx89.rcgu.o: error adding symbols: file in wrong format

If you don't want to solve this issue, then I'm okay with simply having it disabled for now.

[k0tran]

On my loongarch64 machine and cross 0.2.5 (on x86 with loongarch64 toolchain) compiles just fine. I'll look if I can update ci cross version

[k0tran]

After installing rust from rustup everything started failing as in CI. Cross version is the same (0.2.5). I think the main cause of the error is this warning:

[cross] warning: `cross` does not provide a Docker image for target loongarch64-unknown-linux-gnu, specify a custom image in `Cross.toml`.

[k0tran]

Found the problem. Right now cross version from cargo install cross does not work, but master (cargo install cross --git https://github.com/cross-rs/cross) does, so CI should wait until next cross-rs release, I suppose.