Re: [PATCH] Semihost SYS_READC implementation (v4)

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

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

> I'm going to push for somebody actually writing out a
> document and putting it somewhere that we can point to
> and say "that's the authoritative spec", please...
> it doesn't have to be a big formal thing, but I do
> think you want it written down, because the whole point
> is for multiple implementations and users to interoperate.

That happened in June -- I was just looking at the wrong version of the
spec. In the current version, which can be found here:

        https://riscv.org/specifications/

                   The RISC-V Instruction Set Manual
                       Volume I: Unprivileged ISA
                Document Version 20190608-Base-Ratified
                                    
Section 2.8 says:

        Another use of EBREAK is to support “semihosting”, where the
        execution environment includes a debugger that can provide
        services over an alternate system call interface built around
        the EBREAK instruction.  Because the RISC-V base ISA does not
        provide more than one EBREAK instruction, RISC-V semihosting
        uses a special sequence of instructions to distinguish a
        semihosting EBREAK from a debugger inserted EBREAK.

                slli x0, x0, 0x1f   # Entry NOP
                ebreak              # Break to debugger
                srai x0, x0, 7      # NOP encoding the semihosting call number 7

        Note that these three instructions must be 32-bit-wide
        instructions, i.e., they mustn’t be among the compressed 16-bit
        instructions described in Chapter 16.

        The shift NOP instructions are still considered available for
        use as HINTS.

        Semihosting is a form of service call and would be more
        naturally encoded as an ECALL using an existing ABI, but this
        would require the debugger to be able to intercept ECALLs, which
        is a newer addition to the debug standard.  We intend to move
        over to using ECALLs with a standard ABI, in which case,
        semihosting can share a service ABI with an existing standard.

        We note that ARM processors have also moved to using SVC instead
        of BKPT for semihosting calls in newer designs.

I'd like to get the READC patch landed and then post the RISC-V patch
afterwards as the RISC-V patch currently includes READC support.

-- 
-keith

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