Re: [PATCH v2 3/8] machine: Print supported CPU models instead of typenames

[Igor Mammedov <imammedo@redhat.com>]

On Thu, 27 Jul 2023 15:16:18 +1000
Gavin Shan <gshan@redhat.com> wrote:

> On 7/27/23 09:08, Richard Henderson wrote:
> > On 7/25/23 17:32, Gavin Shan wrote:  
> >> -static const char *q800_machine_valid_cpu_types[] = {
> >> +static const char * const q800_machine_valid_cpu_types[] = {
> >>       M68K_CPU_TYPE_NAME("m68040"),
> >>       NULL
> >>   };
> >> +static const char * const q800_machine_valid_cpu_models[] = {
> >> +    "m68040",
> >> +    NULL
> >> +};  
> > 
> > I really don't like this replication.
> >   
> 
> Right, it's going to be lots of replications, but gives much flexibility.
> There are 21 targets and we don't have fixed pattern for the mapping between
> CPU model name and CPU typename. I'm summarizing the used patterns like below.
> 
>    1 All CPU model names are mappinged to fixed CPU typename;

        plainly spelled it would be: cpu_model name ignored and
        a cpu type is returned anyways.

I'd make this hard error right away, as "junk in => error out"
it's clearly user error. I think we don't even have to follow
deprecation process for that.

>    2 CPU model name is same to CPU typename;
>    3 CPU model name is alias to CPU typename;
>    4 CPU model name is prefix of CPU typename;

and some more:
    5. cpu model names aren't names at all sometimes, and some other
       CPU property is used. (ppc)
This one I'd prefer to get rid of and ppc handling more consistent
with other targets, which would need PPC folks to persuaded to drop
PVR lookup.

> 
>    Target         Categories    suffix-of-CPU-typename
>    -------------------------------------------------------
>    alpha          -234          -alpha-cpu
>    arm            ---4          -arm-cpu
>    avr            -2--
>    cris           --34          -cris-cpu
>    hexagon        ---4          -hexagon-cpu
>    hppa           1---
>    i386           ---4          -i386-cpu
>    loongarch      -2-4          -loongarch-cpu
>    m68k           ---4          -m68k-cpu
>    microblaze     1---
>    mips           ---4          -mips64-cpu  -mips-cpu
>    nios2          1---
>    openrisc       ---4          -or1k-cpu
>    ppc            --34          -powerpc64-cpu  -powerpc-cpu
>    riscv          ---4          -riscv-cpu
>    rx             -2-4          -rx-cpu
>    s390x          ---4          -s390x-cpu
>    sh4            --34          -superh-cpu
>    sparc          -2--
>    tricore        ---4          -tricore-cpu
>    xtensa         ---4          -xtensa-cpu
> 
> There are several options as below. Please let me know which one or something
> else is the best.
> 
> (a) Keep what we have and use mc->valid_{cpu_types, cpu_models}[] to track
> the valid CPU typenames and CPU model names.
> 
> (b) Introduce CPUClass::model_name_by_typename(). Every target has their own
> implementation to convert CPU typename to CPU model name. The CPU model name
> is parsed from mc->valid_cpu_types[i].
> 
>      char *CPUClass::model_by_typename(const char *typename);
> 
> (c) As we discussed before, use mc->valid_cpu_type_suffix and mc->valid_cpu_models
> because the CPU type check is currently needed by target arm/m68k/riscv where we
> do have fixed pattern to convert CPU model names to CPU typenames. The CPU typename
> is comprised of CPU model name and suffix. However, it won't be working when the CPU
> type check is required by other target where we have patterns other than this.

none of above is really good, that's why I was objecting to introducing
reverse type->name mapping. That ends up with increased amount junk,
and it's not because your patches are bad, but because you are trying
to deal with cpu model names (which is a historically evolved mess).
The best from engineering POV would be replacing CPU models with
type names.

Even though it's a bit radical, I very much prefer replacing
cpu_model names with '-cpu type'usage directly. Making it
consistent with -device/other interfaces and coincidentally that
obsoletes need in reverse name mapping.

It's painful for end users who will need to change configs/scripts,
but it's one time job. Additionally from QEMU pov, codebase
will be less messy => more maintainable which benefits not only
developers but end-users in the end.

[rant:
It's the same story repeating over and over, when it comes to
changing QEMU CLI, which hits resistance wall. But with QEMU
deprecation process we've changed CLI behavior before,
despite of that world didn't cease to exist and users
have adapted to new QEMU and arguably QEMU became a tiny
bit more maintainable since we don't have to deal some
legacy behavior.
]

Another idea back in the days was (as a compromise),
 1. keep using keep valid_cpu_types
 2. instead of introducing yet another way to do reverse mapping,
    clean/generalize/make it work everywhere list_cpus (which
    already does that mapping) and then use that to do your thing.
    It will have drawbacks you've listed above, but hopefully
    that will clean up and reuse existing list_cpus.
    (only this time, I'd build it around  query-cpu-model-expansion,
     which output is used by generic list_cpus)
    [and here I'm asking to rewrite directly unrelated QEMU part yet again]

> Thanks,
> Gavin
>