Re: [RFC PATCH 00/40] Toward class init of cpu features

[Peter Maydell]

On Fri, 6 Jan 2023 at 19:29, Richard Henderson
<richard.henderson@linaro.org> wrote:
>
> On 1/6/23 11:12, Peter Maydell wrote:
> > The trouble with this idea is that not all instances of the same
> > class are actually necessarily the same. For instance, if you
> > have a system with both (a) a Cortex-A53 with a PMU, and
> > (b) a Cortex-A53 without a PMU, then they're both instances of
> > the same class, but they shouldn't be sharing the coprocessor
> > register hashtable because they don't have an identical set of
> > system registers.
> >
> > This kind of same-CPU-type-heterogenous-configuration system is
> > not something we're currently using on A-profile, but we do have
> > it for M-profile (the sse200 has a dual-core setup where only one
> > of the CPUs has an FPU), so it's not totally outlandish.
>
> Yes, I know.  See patch 29 where I moved the vfp and dsp properties off of 
> the m-profile
> cpus and created new cpu classes instead, specifically for the sse220.
>
> It's not scalable, I'll grant you, but it's hard to design for something 
> we're not using.
> What we use now, apart from the sse200, are common properties set on the 
> command-line.

We also set some properties in code -- eg aspeed_ast2600.c clears
the 'neon' property on its CPUs, lots of the boards clear
has_el3 and has_el2, etc. I hadn't got as far as patch 29, but
looking at it now that looks like a pretty strong indication
that this is the wrong way to go. It creates 3 extra
cortex-m33 CPU classes, and if we find another thing that
ought to be a CPU property then we'll be up to 8; and the
mess propagates up into the SSE-200 class, which is also
no longer able to be configured in the normal way by setting
properties on it, and it becomes visible in user-facing
command line stuff.

I think our object model pretty strongly wants "create object;
set properties on it that only affect this object you created;
realize it", and having one particular subset of objects that
doesn't work the same way is going to be very confusing.

thanks
-- PMM