Re: [Qemu-devel] [PATCH 2/7] target/openrisc: add shutdown logic

[Stafford Horne <shorne@gmail.com>]

On Wed, Apr 27, 2022 at 07:47:33PM +0100, Peter Maydell wrote:
> On Wed, 27 Apr 2022 at 18:46, Jason A. Donenfeld <Jason@zx2c4.com> wrote:
> >
> > Hey Stafford,
> >
> > On Mon, Apr 17, 2017 at 08:23:51AM +0900, Stafford Horne wrote:
> > > In openrisc simulators we use hooks like 'l.nop 1' to cause the
> > > simulator to exit.  Implement that for qemu too.
> > >
> > > Reported-by: Waldemar Brodkorb <wbx@openadk.org>
> > > Signed-off-by: Stafford Horne <shorne@gmail.com>
> >
> > I'm curious as to why this never got merged. I noticed I'm entirely able
> > to shutdown or to reboot (which is mostly what I care about) Linux from
> > OpenRISC. It just hangs.
> 
> This kind of thing needs to be either:
>  (1) we're modelling real hardware and that real hardware has a
> device or other mechanism guest code can prod to cause a power-off
> or reboot. Then we model that device, and guest code triggers a
> shutdown or reboot exactly as it would on the real hardware.
>  (2) there is an architecturally defined ABI for simulators, debug
> stubs, etc, that includes various operations typically including
> an "exit the simulator" function. (Arm semihosting is an example
> of this.) In that case we can implement that functionality,
> guarded by and controlled by the appropriate command line options.
> (This is generally not as nice as option 1, because the guest code
> has to be compiled to have support for semihosting and also because
> turning it on is usually also giving implicit permission for the
> guest code to read and write arbitrary host files, etc.)
> 
> Either way, undocumented random hacks aren't a good idea, which
> is why this wasn't merged.

Yes, this is what was brought up before.  At that time semihosting was mentioned
and I tried to understand what it was but didn't really understand it as a general
concept.  Is this something arm specific?

Since the qemu or1k-sim defines our "simulator", I suspect I could add a
definition of our simulator ABI to the OpenRISC architecture specification.  The
simulation uses of l.nop N as ABI hooks is a de-facto standard for OpenRISC.
From the way you describe this now I take it if we document this as a
architecture simulation ABI the patch would be accepted.

-Stafford