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

[Eduardo Habkost]

On Tue, Mar 28, 2017 at 01:26:04PM +0200, Alexander Graf wrote:
> On 03/28/2017 02:41 AM, Eduardo Habkost wrote:
> > 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?
> 
> Putting it into a special enum sounds much more fragile than the current
> solution to me. We need to bool fallback either way, so I fail to see any
> benefit from having the enum.

I don't see why the enum would be more fragile. With the QAPI
enum, we:
* Have a meaningful value for the QOM property 'type' field,
  and have some hope to make type information for QOM properties
  really useful one day;
* Have the possible values for the property well-documented in
  the QAPI schema;
* Have the string<->enum translation code automatically generated
  for us;
* Can easily add other values later (I have been planning to
  support "feat=host" so "-cpu host/max" aren't special cases in
  the code.

-- 
Eduardo