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

[Igor Mammedov]

On Mon, 31 Jul 2023 15:07:30 +1000
Gavin Shan <gshan@redhat.com> wrote:

> On 7/27/23 19:00, Igor Mammedov wrote:
> > 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.
> >   
> 
> Right, It's not expected behavior to map ambiguous CPU model names to
> the fixed CPU typename.

to be nice we can deprecate those and then later remove.
(while deprecating make those targets accept typenames)

> 
> >>     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.
> >   
> 
> I put this into class 3, meaning the PVRs are regarded as aliases to CPU
> typenames.

with PPC using 'true' aliases, -cpu input is lost after it's translated into 
typename.
(same for alpha)

it also adds an extra fun with 'max' cpu model but that boils down to above 
statement.
(same for
  * sh4
  * cris(in user mode only, but you are making sysemu extension, so it doesn't 
count)
)
For this class of aliases reverse translation won't yield the same
result as used -cpu. The only option you have is to store -cpu cpu_model
somewhere (use qemu_opts??, and then fetch it later to print in error message)

x86 has 'aliases' as well, but in reality it creates distinct cpu types
for each 'alias', so it's possible to do reverse translation.

> >>
> >>     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--

it's case 4

> >>     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.
> >   
> 
> I have to clarify the type->model mapping has been existing since the
> model->type mapping was introduced with the help of CPU_RESOLVING_TYPE.
> I mean the logic has been existing since the existence of CPU_RESOLVING_TYPE,
> even the code wasn't there.
> 
> I'm not sure about the idea to switch to '-cpu <cpu-type-name>' since
> it was rejected by Peter Maydell before. Hope Peter can double confirm
> for this. For me, the shorter name is beneficial. For example, users
> needn't to have '-cpu host-arm-cpu' for '-cpu host'.
> 
> 
> > [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.
> > ]
> >   
> 
> I need more context about 'deprecation process' here. My understanding
> is both CPU typename and model name will be accepted for a fixed period
> of time. However, a warning message will be given to indicate that the
> model name will be obsoleted soon. Eventually, we switch to CPU typename
> completely. Please correct me if there are anything wrong.

yep, that's the gist of deprecation in this case. 
 
> > 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]
> >   
> 
> I'm afraid that list_cpus() is hard to be reused. All available CPU model 
> names
> are listed by list_cpus(). mc->valid_cpu_types[] are just part of them and 
> variable
> on basis of boards. Generally speaking, we need a function to do reverse 
> things
> as to CPUClass::class_by_name(). So I would suggest to introduce 
> CPUClass::model_from_type(),
> as below. Could you please suggest if it sounds reasonable to you?
> 
> - CPUClass::class_by_name() is modified to
>    char *CPUClass::model_to_type(const char *model)
> 
> - char *CPUClass::type_to_model(const char *type)
> 
> - CPUClass::type_to_model() is used in cpu_list() for every target when CPU
>    model name, fetched from CPU type name, is printed in xxx_cpu_list_entry()
> 
> - CPUClass::type_to_model() is reused in hw/core/machine.c to get the CPU
>    model name from CPU type names in mc->valid_cpu_types[].

instead of per target hooks (which are atm mostly open-coded in several places)
how about adding generic handler for cases 2,4:
  cpu_type_to_model(typename)
     cpu_suffix = re'-*-cpu'
     if (class_exists(typename - cpu_suffix))
         return typename - cpu_suffix
     else if (class_exists(typename))
         return typename
     explode

that should work for translating valid_cpus typenames to cpumodel names
and once that in place cleanup all open-coded translations with it tree-wide

you can find those easily by:
git grep _CPU_TYPE_SUFFIX
git grep query_cpu_definitions

> 
> Thanks,
> Gavin
>