Re: [Qemu-devel] [PATCH] target-i386: Disable cache info passthrough by default

[Eduardo Habkost <ehabkost@redhat.com>]

On Tue, Sep 01, 2015 at 02:39:00PM +0200, Igor Mammedov wrote:
> On Wed, 19 Aug 2015 10:08:22 -0700
> Eduardo Habkost <ehabkost@redhat.com> wrote:
> 
> > The host cache information may not make sense for the guest if the VM
> > CPU topology doesn't match the host CPU topology. To make sure we won't
> > expose broken cache information to the guest, disable cache info
> > passthrough by default, and add a new "host-cache-info" property that
> > can be used to enable the old behavior for users that really need it.
> > 
> > Cc: Benoît Canet <benoit@irqsave.net>
> > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > ---
> >  target-i386/cpu.c | 4 +---
> >  1 file changed, 1 insertion(+), 3 deletions(-)
> > 
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index cfb8aa7..3a71f15 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -689,7 +689,6 @@ struct X86CPUDefinition {
> >      int stepping;
> >      FeatureWordArray features;
> >      char model_id[48];
> > -    bool cache_info_passthrough;
> >  };
> >  
> >  static X86CPUDefinition builtin_x86_defs[] = {
> > @@ -1416,6 +1415,7 @@ static X86CPUDefinition host_cpudef;
> >  
> >  static Property host_x86_cpu_properties[] = {
> >      DEFINE_PROP_BOOL("migratable", X86CPU, migratable, true),
> > +    DEFINE_PROP_BOOL("host-cache-info", X86CPU, cache_info_passthrough, false),
> >      DEFINE_PROP_END_OF_LIST()
> >  };
> >  
> > @@ -1442,7 +1442,6 @@ static void host_x86_cpu_class_init(ObjectClass *oc, void *data)
> >      cpu_x86_fill_model_id(host_cpudef.model_id);
> >  
> >      xcc->cpu_def = &host_cpudef;
> > -    host_cpudef.cache_info_passthrough = true;
> >  
> >      /* level, xlevel, xlevel2, and the feature words are initialized on
> >       * instance_init, because they require KVM to be initialized.
> > @@ -2076,7 +2075,6 @@ static void x86_cpu_load_def(X86CPU *cpu, X86CPUDefinition *def, Error **errp)
> >      object_property_set_int(OBJECT(cpu), def->stepping, "stepping", errp);
> >      object_property_set_int(OBJECT(cpu), def->xlevel, "xlevel", errp);
> >      object_property_set_int(OBJECT(cpu), def->xlevel2, "xlevel2", errp);
> > -    cpu->cache_info_passthrough = def->cache_info_passthrough;
> Isn't that a guest visible change?

It is. I am not convinced we should spend time trying to make live
migration work with -cpu host unless we hear from an actual user that
depend on it, but in this case it is really simple to keep compatibility
by adding host-cache-info=on to PC_COMPAT_2_4, so I will send a new
version.

> 
> >      object_property_set_str(OBJECT(cpu), def->model_id, "model-id", errp);
> >      for (w = 0; w < FEATURE_WORDS; w++) {
> >          env->features[w] = def->features[w];
> 

-- 
Eduardo