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Re: [RFC PATCH v3 1/5] ppc64: Add semihosting support

From: Peter Maydell
Subject: Re: [RFC PATCH v3 1/5] ppc64: Add semihosting support
Date: Wed, 20 Apr 2022 20:24:26 +0100

On Wed, 20 Apr 2022 at 19:20, Leandro Lupori
<leandro.lupori@eldorado.org.br> wrote:
> On 4/19/22 06:26, Peter Maydell wrote:
> > On Mon, 18 Apr 2022 at 20:15, Leandro Lupori
> > <leandro.lupori@eldorado.org.br> wrote:
> >>
> >> Add semihosting support for PPC64. This implementation is
> >> based on the standard for ARM semihosting version 2.0, as
> >> implemented by QEMU and documented in
> >>
> >>      https://github.com/ARM-software/abi-aa/releases
> >>
> >> The PPC64 specific differences are the following:
> >>
> >> Semihosting Trap Instruction: sc 7
> >> Operation Number Register: r3
> >> Parameter Register: r4
> >> Return Register: r3
> >> Data block field size: 64 bits
> >
> > Where is the independent specification which defines that
> > this is the ABI for PPC semihosting? You should provide the
> > URL for that in a comment somewhere.
> >
> AFAIK, there is no official PPC semihosting specification. Would it be
> ok to just document it somewhere else, e.g. GitHub, as an unofficial
> specification?

I'm going to push back on this in the same way I did for
the RISC-V folks. If this is an official PPC semihosting
specification, intended to be supported by multiple
different pieces of software, it needs to have an
independent spec document somewhere (even if that
spec document just cross-refers to the Arm spec for
most of the detail). If this is an ad-hoc "add this
thing for PPC in a purely QEMU-specific way" patchset,
then no, we shouldn't implement it.

Semihosting is an ABI, and when QEMU implements an ABI
it should be because it's an external pre-existing one.

-- PMM

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