[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [PATCH] Semihost SYS_READC implementation (v4)
From: |
Peter Maydell |
Subject: |
Re: [PATCH] Semihost SYS_READC implementation (v4) |
Date: |
Mon, 11 Nov 2019 14:51:06 +0000 |
On Tue, 5 Nov 2019 at 05:10, Keith Packard <address@hidden> wrote:
>
> Peter Maydell <address@hidden> 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.
That defines the instruction sequence used to make a semihosting
call, but not the specification of what the calls are:
* what call numbers perform which functions
* how arguments are passed to the call (registers? parameter
blocks in memory? other?)
* the semantics of each function supported (number of arguments,
behaviour, error handling)
That's really what I had in mind by the overall semihosting spec.
PS: the parenthetical about ARM semihosting at the bottom of
the text you quote is wrong, incidentally. The traditional insn
for semihosting on A-profile devices has always been SWI/SVC; it
is BKPT only on M-profile devices; and the latest revision of the
semihosting spec recommends the HLT instruction for both A- and M-.
thanks
-- PMM