Re: [PATCH v2 1/2] Introduce (x86) CPU model deprecation API

[Robert Hoo]

On Wed, 2020-09-09 at 14:15 -0400, Eduardo Habkost wrote:
> Hi,
> 
> Thanks for the patch, and sorry for taking so long to review
> this.  I'm finally getting to the patches that were postponed to
> 5.2.
> 
> Comments and questions below:
> 
> On Thu, Jun 11, 2020 at 10:47:55AM +0800, Robert Hoo wrote:
> > Complement versioned CPU model framework with the ability of
> > marking some
> > versions deprecated. When that CPU model is chosen, get some
> > warning. The
> > warning message is customized, e.g. telling in which future QEMU
> > version will
> > it be obsoleted.
> > The deprecation message will also appear by x86_cpu_list_entry(),
> > e.g. '-cpu
> > help'.
> > QMP 'query-cpu-definitions' will also return a bool value
> > indicating the
> > deprecation status.
> > 
> > Changes in v2:
> > Move deprecation check from parse_cpu_option() to
> > machine_run_board_init(), so
> > that it can cover implicit cpu_type assignment cases.
> > Add qapi new member documentation. Thanks Eric for comment and
> > guidance on qapi.
> > 
> > Signed-off-by: Robert Hoo <robert.hu@linux.intel.com>
> > ---
> >  hw/core/machine.c        | 11 +++++++++--
> >  include/hw/core/cpu.h    |  1 +
> >  qapi/machine-target.json |  7 ++++++-
> >  target/i386/cpu.c        | 45
> > +++++++++++++++++++++++++++++++++++++++++++--
> >  4 files changed, 59 insertions(+), 5 deletions(-)
> > 
> > diff --git a/hw/core/machine.c b/hw/core/machine.c
> > index bb3a7b1..9318964 100644
> > --- a/hw/core/machine.c
> > +++ b/hw/core/machine.c
> > @@ -1083,6 +1083,8 @@ MemoryRegion
> > *machine_consume_memdev(MachineState *machine,
> >  void machine_run_board_init(MachineState *machine)
> >  {
> >      MachineClass *machine_class = MACHINE_GET_CLASS(machine);
> > +    ObjectClass *oc = object_class_by_name(machine->cpu_type);
> > +    CPUClass *cc;
> >  
> >      if (machine->ram_memdev_id) {
> >          Object *o;
> > @@ -1102,11 +1104,10 @@ void machine_run_board_init(MachineState
> > *machine)
> >       * specified a CPU with -cpu check here that the user CPU is
> > supported.
> >       */
> >      if (machine_class->valid_cpu_types && machine->cpu_type) {
> > -        ObjectClass *class = object_class_by_name(machine-
> > >cpu_type);
> >          int i;
> >  
> >          for (i = 0; machine_class->valid_cpu_types[i]; i++) {
> > -            if (object_class_dynamic_cast(class,
> > +            if (object_class_dynamic_cast(oc,
> >                                            machine_class-
> > >valid_cpu_types[i])) {
> >                  /* The user specificed CPU is in the valid field,
> > we are
> >                   * good to go.
> > @@ -1129,6 +1130,12 @@ void machine_run_board_init(MachineState
> > *machine)
> >          }
> >      }
> >  
> > +    /* Check if CPU type is deprecated and warn if so */
> > +    cc = CPU_CLASS(oc);
> > +    if (cc->deprecation_check) {
> > +        cc->deprecation_check(oc);
> > +    }
> 
> Why do we need target-specific code here?  A CPUClass::deprecated
> field would be much simpler.
> 
Because the Warning message composing is target-specific, using
X86CPUVersionDefinition.note.
Other targets can have their own warning message composing approaches.

> > +
> >      machine_class->init(machine);
> >  }
> >  
> > diff --git a/include/hw/core/cpu.h b/include/hw/core/cpu.h
> > index 497600c..1ca47dc 100644
> > --- a/include/hw/core/cpu.h
> > +++ b/include/hw/core/cpu.h
> > @@ -218,6 +218,7 @@ typedef struct CPUClass {
> >      void (*disas_set_info)(CPUState *cpu, disassemble_info *info);
> >      vaddr (*adjust_watchpoint_address)(CPUState *cpu, vaddr addr,
> > int len);
> >      void (*tcg_initialize)(void);
> > +    void (*deprecation_check)(ObjectClass *oc);
> >  
> >      /* Keep non-pointer data at the end to minimize holes.  */
> >      int gdb_num_core_regs;
> > diff --git a/qapi/machine-target.json b/qapi/machine-target.json
> > index f2c8294..c24f506 100644
> > --- a/qapi/machine-target.json
> > +++ b/qapi/machine-target.json
> > @@ -285,6 +285,10 @@
> >  #            in the VM configuration, because aliases may stop
> > being
> >  #            migration-safe in the future (since 4.1)
> >  #
> > +# @deprecated: If true, this CPU model is deprecated and may be
> > removed in
> > +#              in some future version of QEMU according to the
> > QEMU deprecation
> > +#              policy. (since 5.1)
> 
> Next version needs to say "since 5.2".

Sure.
> 
> > +#
> >  # @unavailable-features is a list of QOM property names that
> >  # represent CPU model attributes that prevent the CPU from
> > running.
> >  # If the QOM property is read-only, that means there's no known
> > @@ -309,7 +313,8 @@
> >              'static': 'bool',
> >              '*unavailable-features': [ 'str' ],
> >              'typename': 'str',
> > -            '*alias-of' : 'str' },
> > +            '*alias-of' : 'str',
> > +            'deprecated' : 'bool' },
> >    'if': 'defined(TARGET_PPC) || defined(TARGET_ARM) ||
> > defined(TARGET_I386) || defined(TARGET_S390X) ||
> > defined(TARGET_MIPS)' }
> >  
> >  ##
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index ba05da3..0d8638a 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -1599,6 +1599,7 @@ typedef struct X86CPUVersionDefinition {
> >      const char *alias;
> >      const char *note;
> >      PropValue *props;
> > +    bool       deprecated;
> >  } X86CPUVersionDefinition;
> >  
> >  /* Base definition for a CPU model */
> > @@ -1638,6 +1639,11 @@ struct X86CPUModel {
> >       * This matters only for "-cpu help" and query-cpu-definitions
> >       */
> >      bool is_alias;
> > +    /*
> > +     * If true, this model is deprecated, and may be removed in
> > the future.
> > +     * Trying to use it now will cause a warning.
> > +     */
> > +    bool deprecated;
> >  };
> >  
> >  /* Get full model name for CPU version */
> > @@ -4128,8 +4134,7 @@ static X86CPUVersion
> > x86_cpu_model_resolve_version(const X86CPUModel *model)
> >      X86CPUVersion v = model->version;
> >      if (v == CPU_VERSION_AUTO) {
> >          v = default_cpu_version;
> > -    }
> > -    if (v == CPU_VERSION_LATEST) {
> > +    } else if (v == CPU_VERSION_LATEST) {
> 
> Why is this change necessary?

Just kind of compulsion of avoiding unnecessary if() :-). 'v' can only
be one of CPU_VERSION_AUTO and CPU_VERSION_LATEST, unnecessary to judge
twice.
> 
> >          return x86_cpu_model_last_version(model);
> >      }
> >      return v;
> > @@ -4975,6 +4980,7 @@ static void x86_cpu_definition_entry(gpointer
> > data, gpointer user_data)
> >      info->migration_safe = cc->migration_safe;
> >      info->has_migration_safe = true;
> >      info->q_static = cc->static_model;
> > +    info->deprecated = cc->model ? cc->model->deprecated : false;
> >      /*
> >       * Old machine types won't report aliases, so that alias
> > translation
> >       * doesn't break compatibility with previous QEMU versions.
> > @@ -5411,6 +5417,7 @@ static void
> > x86_register_cpudef_types(X86CPUDefinition *def)
> >          m->cpudef = def;
> >          m->version = vdef->version;
> >          m->note = vdef->note;
> > +        m->deprecated = vdef->deprecated;
> >          x86_register_cpu_model_type(name, m);
> >  
> >          if (vdef->alias) {
> > @@ -5418,6 +5425,8 @@ static void
> > x86_register_cpudef_types(X86CPUDefinition *def)
> >              am->cpudef = def;
> >              am->version = vdef->version;
> >              am->is_alias = true;
> > +            am->note = vdef->note;
> 
> Is this extra line related to the deprecation feature?
> 
> It doesn't seem related, and it doesn't seem necessary as the
> `note` field is already ignored for alias CPU models.

Because it is unused by other features, I use it to store model
specific deprecation message.
> 
> > +            am->deprecated = vdef->deprecated;
> >              x86_register_cpu_model_type(vdef->alias, am);
> >          }
> >      }
> > @@ -7233,6 +7242,37 @@ static Property x86_cpu_properties[] = {
> >      DEFINE_PROP_END_OF_LIST()
> >  };
> >  
> > +static void x86_cpu_deprecation_check(ObjectClass *oc)
> > +{
> > +    X86CPUClass *xcc = X86_CPU_CLASS(oc);
> > +    X86CPUVersion effective_version;
> > +    const X86CPUVersionDefinition *vdef;
> > +
> > +    if (xcc->model == NULL) {
> > +        return;
> > +    }
> > +
> > +    if (xcc->model->version == CPU_VERSION_LEGACY) {
> > +        /* Treat legacy version as v1 */
> > +        effective_version = 1;
> > +    } else {
> > +        effective_version = x86_cpu_model_resolve_version(xcc-
> > >model);
> > +    }
> > +
> > +    vdef = xcc->model->cpudef->versions;
> > +
> > +    if (vdef == NULL) {
> > +        return;
> > +    } else {
> > +        if (vdef[effective_version - 1].deprecated) {
> > +            warn_report("Effective CPU model '%s' -- %s",
> > +                    x86_cpu_versioned_model_name(xcc->model-
> > >cpudef,\
> > +                                                effective_version)
> > ,
> > +                    vdef[effective_version - 1].note);
> > +        }
> > +    }
> 
> Why do we need this extra logic?  Isn't it simpler to just add a
> bool CPUClass::deprecated field, and set:
> 
>    cpu->deprecated = model->deprecated;
> 
> inside x86_cpu_cpudef_class_init()?
> 
All these are to fulfill the target you expected earlier:

"We need a proper CPU model deprecation API.  Deprecation info
should appear on query-cpu-definitions and should trigger a
warning when using the CPU model."

So I think each deprecated model shall have its own deprecation
message, e.g. by which version it's going to be deprecation, etc.

> > +}
> > +
> >  static void x86_cpu_common_class_init(ObjectClass *oc, void *data)
> >  {
> >      X86CPUClass *xcc = X86_CPU_CLASS(oc);
> > @@ -7291,6 +7331,7 @@ static void
> > x86_cpu_common_class_init(ObjectClass *oc, void *data)
> >      cc->tlb_fill = x86_cpu_tlb_fill;
> >  #endif
> >      cc->disas_set_info = x86_disas_set_info;
> > +    cc->deprecation_check = x86_cpu_deprecation_check;
> >  
> >      dc->user_creatable = true;
> >  }
> > -- 
> > 1.8.3.1
> > 
> 
>