Re: [PATCH] Semihost SYS_READC implementation (v4)

["Keith Packard" <keithp@keithp.com>]

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Peter Maydell <peter.maydell@linaro.org> writes:

> Is there a specification for RISC-V semihosting? This is
> likely to be my first question when the support comes
> round for review, so you can have it early :-)  We'd
> prefer to implement specified interfaces, not random
> ad-hoc "this seems to be what newlib wants to see,
> which is turn got hacked together by copying some other
> architecture's code".

There seems to be convergence on a pretty simple interface which uses
ebreak surrounded by a couple of specific no-ops:

      slli x0, x0, 0x1f
      ebreak
      srai x0, x0, 0x7

There are implementations in rust and openocd, and I've got one for
picolibc. The risc-v semihosting code is sitting on a branch in my repo
on github:

        https://github.com/keith-packard/qemu/tree/riscv-semihost

> (describing a mechanism to avoid stopping the emulator)
> This feels to me like it's a bit overcomplicated unless it turns out
> we actually require it though.

Would also be nice for multi-core setups. I'd like to start with the
simple plan for now.

> Isn't the answer to this "don't use a command line that tries
> to connect stdio to multiple things" ?

Uh, we do that all the time? The mux device is designed to handle this
so that you can use stdio for both monitor commands and application
I/O. It's very convenient, the only issue is that the last device that
hooks to the mux ends up getting input first (you use ^Ac to rotate
among the selected devices).

-- 
-keith

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