Re: [Qemu-devel] [PATCH] i386: Allow cpuid bit override

[Eduardo Habkost]

On Tue, Mar 28, 2017 at 12:19:37AM +0200, Alexander Graf wrote:
> KVM has a feature bitmap of CPUID bits that it knows works for guests.
> QEMU removes bits that are not part of that bitmap automatically on VM
> start.
> 
> However, some times we just don't list features in that list because
> they don't make sense for normal scenarios, but may be useful in specific,
> targeted workloads.
> 
> For that purpose, add a new =force option to all CPUID feature flags in
> the CPU property. With that we can override the accel filtering and give
> users full control over the CPUID feature bits exposed into guests.
> 
> Signed-off-by: Alexander Graf <address@hidden>
> ---
>  target/i386/cpu.c | 30 ++++++++++++++++++++++++++----
>  target/i386/cpu.h |  3 +++
>  2 files changed, 29 insertions(+), 4 deletions(-)
> 
> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> index 7aa7622..5a22f9a 100644
> --- a/target/i386/cpu.c
> +++ b/target/i386/cpu.c
> @@ -2229,7 +2229,7 @@ void x86_cpu_list(FILE *f, fprintf_function cpu_fprintf)
>      g_slist_foreach(list, x86_cpu_list_entry, &s);
>      g_slist_free(list);
>  
> -    (*cpu_fprintf)(f, "\nRecognized CPUID flags:\n");
> +    (*cpu_fprintf)(f, "\nRecognized CPUID flags (=on|=off|=force):\n");
>      for (i = 0; i < ARRAY_SIZE(feature_word_info); i++) {
>          FeatureWordInfo *fw = &feature_word_info[i];
>  
> @@ -3460,6 +3460,7 @@ static int x86_cpu_filter_features(X86CPU *cpu)
>              x86_cpu_get_supported_feature_word(w, false);
>          uint32_t requested_features = env->features[w];
>          env->features[w] &= host_feat;
> +        env->features[w] |= cpu->forced_features[w];
>          cpu->filtered_features[w] = requested_features & ~env->features[w];
>          if (cpu->filtered_features[w]) {
>              rv = 1;
> @@ -3693,6 +3694,7 @@ static void x86_cpu_unrealizefn(DeviceState *dev, Error 
> **errp)
>  
>  typedef struct BitProperty {
>      uint32_t *ptr;
> +    uint32_t *force_ptr;
>      uint32_t mask;
>  } BitProperty;

Please take a look at:
  Subject: [PATCH for-2.9 v2 1/2] i386: Replace uint32_t* with FeatureWord on 
feature getter/setter

I plan to include that series in 2.9, and it would make the
force_ptr field unnecessary.

>  
> @@ -3701,7 +3703,15 @@ static void x86_cpu_get_bit_prop(Object *obj, Visitor 
> *v, const char *name,
>  {
>      BitProperty *fp = opaque;
>      bool value = (*fp->ptr & fp->mask) == fp->mask;
> -    visit_type_bool(v, name, &value, errp);
> +    bool forced = (*fp->force_ptr & fp->mask) == fp->mask;
> +    char str[] = "force";
> +    char *strval = str;
> +
> +    if (!forced) {
> +        strcpy(str, value ? "on" : "off");
> +    }
> +
> +    visit_type_str(v, name, &strval, errp);

You can define an enum type in qapi-schema.json, and use
visit_type_<YourEnumType>(). You can grep for
visit_type_OnOffAuto to find examples.

(But I suggest naming the enum something like
"X86CPUFeatureSetting" instead of "OnOffForce", because we will
probably add other enum values in the future).

However: we need to find a way to do this and not break
compatibility with "feat=yes|true|no|false", that's supported by
StringInputVisitor (which is used by object_property_parse()).
Maybe fallback to visit_type_bool() in case
visit_type_<YourEnumType>() fails?

Except for that, the proposed logic looks good to me.


>  }
>  
>  static void x86_cpu_set_bit_prop(Object *obj, Visitor *v, const char *name,
> @@ -3710,6 +3720,7 @@ static void x86_cpu_set_bit_prop(Object *obj, Visitor 
> *v, const char *name,
>      DeviceState *dev = DEVICE(obj);
>      BitProperty *fp = opaque;
>      Error *local_err = NULL;
> +    char *strval = NULL;
>      bool value;
>  
>      if (dev->realized) {
> @@ -3717,7 +3728,15 @@ static void x86_cpu_set_bit_prop(Object *obj, Visitor 
> *v, const char *name,
>          return;
>      }
>  
> -    visit_type_bool(v, name, &value, &local_err);
> +    visit_type_str(v, name, &strval, &local_err);
> +    if (!local_err && !strcmp(strval, "force")) {
> +        value = true;
> +        *fp->force_ptr |= fp->mask;
> +    } else {
> +        local_err = NULL;
> +        visit_type_bool(v, name, &value, &local_err);
> +    }
> +
>      if (local_err) {
>          error_propagate(errp, local_err);
>          return;
> @@ -3746,6 +3765,7 @@ static void x86_cpu_release_bit_prop(Object *obj, const 
> char *name,
>  static void x86_cpu_register_bit_prop(X86CPU *cpu,
>                                        const char *prop_name,
>                                        uint32_t *field,
> +                                      uint32_t *force_field,
>                                        int bitnr)
>  {
>      BitProperty *fp;
> @@ -3760,6 +3780,7 @@ static void x86_cpu_register_bit_prop(X86CPU *cpu,
>      } else {
>          fp = g_new0(BitProperty, 1);
>          fp->ptr = field;
> +        fp->force_ptr = force_field;
>          fp->mask = mask;
>          object_property_add(OBJECT(cpu), prop_name, "bool",
>                              x86_cpu_get_bit_prop,
> @@ -3787,7 +3808,8 @@ static void x86_cpu_register_feature_bit_props(X86CPU 
> *cpu,
>      /* aliases don't use "|" delimiters anymore, they are registered
>       * manually using object_property_add_alias() */
>      assert(!strchr(name, '|'));
> -    x86_cpu_register_bit_prop(cpu, name, &cpu->env.features[w], bitnr);
> +    x86_cpu_register_bit_prop(cpu, name, &cpu->env.features[w],
> +                              &cpu->forced_features[w], bitnr);
>  }
>  
>  static GuestPanicInformation *x86_cpu_get_crash_info(CPUState *cs)
> diff --git a/target/i386/cpu.h b/target/i386/cpu.h
> index 07401ad..128530e 100644
> --- a/target/i386/cpu.h
> +++ b/target/i386/cpu.h
> @@ -1228,6 +1228,9 @@ struct X86CPU {
>      /* Features that were filtered out because of missing host capabilities 
> */
>      uint32_t filtered_features[FEATURE_WORDS];
>  
> +    /* Features that are force enabled despite incompatible accel */
> +    uint32_t forced_features[FEATURE_WORDS];
> +
>      /* Enable PMU CPUID bits. This can't be enabled by default yet because
>       * it doesn't have ABI stability guarantees, as it passes all PMU CPUID
>       * bits returned by GET_SUPPORTED_CPUID (that depend on host CPU and 
> kernel
> -- 
> 1.8.5.6
> 

-- 
Eduardo