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



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
> 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
> 
> 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 CONFIG_KVM is not set, kvm_enabled() is always zero, so the function
would never be called, so I find the ifdef-less code more readable and
obvious.

What I don't know is if we should do this:

  #ifdef CONFIG_KVM
  
  static int kvm_check_features_against_host(...)
  {
      /* real implementation here */
  }
  
  static int kvm_do_something_else(...)
  {
      /* real implementation here */
  }
  
  /* Other kvm_* functions here */
  
  #else
  
  static int kvm_check_features_against_host(...)
  {
  }
  
  static int kvm_do_something_else(...)
  {
  }
  
  /* Other kvm_* stubs here */
  
  #endif /* CONFIG_KVM */


Or this:

  static int kvm_check_features_against_host(...)
  {
  #ifdef CONFIG_KVM
      /* real implementation here */
  #endif /* CONFIG_KVM */
  }
  
  static int kvm_do_something_else(...)
  {
  #ifdef CONFIG_KVM
      /* real implementation here */
  #endif /* CONFIG_KVM */
  }


I believe the latter is better, but based on Gleb's comments about
enable_kvm_pv_eoi(), he seems to prefer the former.

-- 
Eduardo


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