Re: Semihosting, arm, riscv, ppc and common code

[Benjamin Herrenschmidt <benh@kernel.crashing.org>]

On Tue, 2020-01-14 at 09:59 +0000, Peter Maydell wrote:
> Note that semihosting is not a "here's a handy QEMU feature"
> thing. It's an architecture-specific API and ABI, which should
> be defined somewhere in a standard external to QEMU.

There is no such standard for powerpc today that I know of.

> You need to start by having a definition for PPC of what
> semihosting is. If you're starting from scratch there, there
> are some important things you should do differently to Arm --
> there is no benefit to repeating the mistakes of API definition
> that we made! Most notably, you want to specify and require
> that any unrecognized semihosting call function fails in a
> clean and detectable way; you also should have a semihosting
> function for "ask for a feature bit mask" so you don't need
> the silly magic-filename approach Arm had to go for. You
> also want to standardize what the errno values are, which Arm
> forgot to do and which makes the errno handling in the spec
> pretty useless.

Keith and I are somewhat of a different mind here. From the perspective
of the user of that API (picolibc is one), it's easier to deal with a
single one and have everybody inherit the same bugs.

Now I understand the point of wanting to fix the mistakes made but I
would suggest we do so by proposing extensions to the existing one to
do so.

> TLDR: don't start by writing code, start by writing the *API/ABI
> spec*.
> I tried to push the RISCV folks in this direction as well.

AFAIK they are still just doing what ARM does for the above reason.

Cheers,
Ben.