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

[Benjamin Herrenschmidt]

On Wed, 2020-01-15 at 12:01 +0000, Alex Bennée wrote:
> 
> > There seem to be some linux-user stuff in there, I'm mostly considering
> > whatever ARM does today but we can certainly extend later.
> 
> Depends on if it is to be used. AFAIK the main users of arm linux user
> are compiler test cases for M-profile CPUs. 

For microwatt I was going to implement HW support as well via JTAG
but the user linux-user bits are less obviously useful to me.

That said, most of that code can trivially be made arch neutral by
replacing the use of the arch specific CPU type with CPUState as
the first argument to most functions. There are only a handful of arch
specific helpers needed from there to extract the op & arg, set the
result etc..

> > The idea is to make sure ARM, RiscV and POWER use the same protocol and
> > code base to make picolibc (and others) life easier. Bug compatible
> > :-)
> 
> Hmm, I'm not so sure. QEMU tries to emulate real HW although I
> appreciate that is a somewhat loose definition once you get to things
> like -M virt and other such SW like abstractions. Is semihosting even
> going to be a thing on real RiscV/Power silicon?

It will be on microwatt once I add support for it. We could probably
make it work on real power9 if the systems give access to the external
debug facilities of the processor as well. I'm no longer involved with
powerpc in a professional capacity but I can ask Anton or Paul to help
there.

Cheers,
Ben.