Re: [Qemu-devel] [PATCH qom-cpu 10/11] target-i386: Call kvm_check_features_against_host() only if CONFIG_KVM is set

[Igor Mammedov <imammedo@redhat.com>]

On Mon, 7 Jan 2013 15:30:26 +0200
Gleb Natapov <gleb@redhat.com> wrote:

> On Mon, Jan 07, 2013 at 02:15:14PM +0100, Igor Mammedov wrote:
> > On Mon, 7 Jan 2013 10:00:09 -0200
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > 
> > > On Sun, Jan 06, 2013 at 04:27:19PM +0200, Gleb Natapov wrote:
> > > > On Fri, Jan 04, 2013 at 08:01:11PM -0200, Eduardo Habkost wrote:
> > > > > This will be necessary once kvm_check_features_against_host() starts
> > > > > using KVM-specific definitions (so it won't compile anymore if
> > > > > CONFIG_KVM is not set).
> > > > > 
> > > > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > > > ---
> > > > >  target-i386/cpu.c | 4 ++++
> > > > >  1 file changed, 4 insertions(+)
> > > > > 
> > > > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > > > index 1c3c7e1..876b0f6 100644
> > > > > --- a/target-i386/cpu.c
> > > > > +++ b/target-i386/cpu.c
> > > > > @@ -936,6 +936,7 @@ static void kvm_cpu_fill_host(x86_def_t *x86_cpu_def)
> > > > >  #endif /* CONFIG_KVM */
> > > > >  }
> > > > >  
> > > > > +#ifdef CONFIG_KVM
> > > > >  static int unavailable_host_feature(struct model_features_t *f, uint32_t mask)
> > > > >  {
> > > > >      int i;
> > > > > @@ -987,6 +988,7 @@ static int kvm_check_features_against_host(x86_def_t *guest_def)
> > > > >                  }
> > > > >      return rv;
> > > > >  }
> > > > > +#endif
> > > > >  
> > > > >  static void x86_cpuid_version_get_family(Object *obj, Visitor *v, void *opaque,
> > > > >                                           const char *name, Error **errp)
> > > > > @@ -1410,10 +1412,12 @@ static int cpu_x86_parse_featurestr(x86_def_t *x86_cpu_def, char *features)
> > > > >      x86_cpu_def->kvm_features &= ~minus_kvm_features;
> > > > >      x86_cpu_def->svm_features &= ~minus_svm_features;
> > > > >      x86_cpu_def->cpuid_7_0_ebx_features &= ~minus_7_0_ebx_features;
> > > > > +#ifdef CONFIG_KVM
> > > > >      if (check_cpuid && kvm_enabled()) {
> > > > >          if (kvm_check_features_against_host(x86_cpu_def) && enforce_cpuid)
> > > > >              goto error;
> > > > >      }
> > > > > +#endif
> > > > Provide kvm_check_features_against_host() stub if !CONFIG_KVM and drop
> > > > ifdef here.
> > > 
> > > I will do. Igor probably will have to change his "target-i386: move
> > > kvm_check_features_against_host() check to realize time" patch to use
> > > the same approach, too.
> > 
> > 
> > Gleb,
> > 
> > Why do stub here? As result we will be adding more ifdef-s just in other
> > places. Currently kvm_cpu_fill_host(), unavailable_host_feature() and
> Why will we be adding more ifdef-s in other places?
unavailable_host_feature() is being ifdef-ed above

> 
> > kvm_check_features_against_host() are bundled together in cpu.c so we could
> > instead ifdef whole block. Like here:
> > http://www.mail-archive.com/qemu-devel@nongnu.org/msg146536.html
> > 
> That's fine, but you can avoid things like:
> 
>      if (kvm_enabled() && name && strcmp(name, "host") == 0) {
> +#ifdef CONFIG_KVM
>          kvm_cpu_fill_host(x86_cpu_def);
> +#endif
> 
> in your patch by providing stub for kvm_cpu_fill_host() for !CONFIG_KVM
> case. This is common practice really. Avoid ifdefs in the code.
This ifdef could be eliminated later when cpus are converted into sub-classes.
Then we would put host subclass close to kvm_cpu_fill_host inside of the same
ifdef. that would leave ifdef around kvm_check_features_against_host() in
cpu_x86_parse_featurestr().

> 
> > For me code looks more readable with ifdef here, if we have stub, a reader
> > would have to look at kvm_check_features_against_host() body to see if it does
> > anything.
> > 
> If reader cares about kvm it has to anyway. If he does not, there is
> friendly kvm_enabled() (which is stub in case of !CONFIG_KVM BTW) to
> tell him that he does not care. No need additional ifdef there.

both ways would work, but if stubs are preferred style then there is no
point arguing.

> 
> --
> 			Gleb.


-- 
Regards,
  Igor