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

From: Benjamin Herrenschmidt
Subject: Re: Semihosting, arm, riscv, ppc and common code
Date: Thu, 16 Jan 2020 13:01:23 +1100

On Wed, 2020-01-15 at 13:32 +0000, Peter Maydell wrote:
> On Wed, 15 Jan 2020 at 01:17, Benjamin Herrenschmidt
> <address@hidden> wrote:
> > 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.
> So you need to write one down somewhere. You're proposing
> an ABI which will be implemented by multiple implementors
> and used by multiple consumers. That needs a spec, not
> just code. I don't want to see more semihosting implementations
> added to QEMU that don't have a specification written
> down somewhere.

That's ok, I can probably get openpower to put a link to the ARM one
somewhere :-)

> The point about the mistakes is that you can't easily fix
> them by adding extensions, because the core of the biggest
> mistake is "we didn't provide a good way to allow extensions to
> be added and probed for by the user". So we had to implement
> an ugly and hard to implement mechanism instead.
>  New
> architectures could mandate providing the good way from the start
> and avoid the painful-to-implement approach entirely.
> (I think RISCV has already missed this window of opportunity,
> which is a shame.)

It is done and so now we have two architectures using that standard, I
think the value in using the same overweight the value in fixing it but
yes, we should try to agree on a method of extending at least. Is it
really that hard ?

IE. We could add a new call for capabilities that takes a pointer to
a region which we pre-zero before calling in the client and if remains
zero after the call, then the new stuff isn't there for example. That
sort of stuff is easy, or am I missing something ?


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