Re: [Qemu-devel] [PATCH qom-cpu 01/11] target-i386: Don't set any KVM flag by default if KVM is disabled

[Gleb Natapov]

On Mon, Jan 07, 2013 at 09:42:36AM -0200, Eduardo Habkost wrote:
> On Sun, Jan 06, 2013 at 01:32:34PM +0200, Gleb Natapov wrote:
> > On Fri, Jan 04, 2013 at 08:01:02PM -0200, Eduardo Habkost wrote:
> > > This is a cleanup that tries to solve two small issues:
> > > 
> > >  - We don't need a separate kvm_pv_eoi_features variable just to keep a
> > >    constant calculated at compile-time, and this style would require
> > >    adding a separate variable (that's declared twice because of the
> > >    CONFIG_KVM ifdef) for each feature that's going to be enabled/disable
> > >    by machine-type compat code.
> > >  - The pc-1.3 code is setting the kvm_pv_eoi flag on cpuid_kvm_features
> > >    even when KVM is disabled at runtime. This small incosistency in
> > >    the cpuid_kvm_features field isn't a problem today because
> > >    cpuid_kvm_features is ignored by the TCG code, but it may cause
> > >    unexpected problems later when refactoring the CPUID handling code.
> > > 
> > > This patch eliminates the kvm_pv_eoi_features variable and simply uses
> > > CONFIG_KVM and kvm_enabled() inside the enable_kvm_pv_eoi() compat
> > > function, so it enables kvm_pv_eoi only if KVM is enabled. I believe
> > > this makes the behavior of enable_kvm_pv_eoi() clearer and easier to
> > > understand.
> > > 
> > > Signed-off-by: Eduardo Habkost <address@hidden>
> > > ---
> > > Cc: address@hidden
> > > Cc: Michael S. Tsirkin <address@hidden>
> > > Cc: Gleb Natapov <address@hidden>
> > > Cc: Marcelo Tosatti <address@hidden>
> > > 
> > > Changes v2:
> > >  - Coding style fix
> > > ---
> > >  target-i386/cpu.c | 8 +++++---
> > >  1 file changed, 5 insertions(+), 3 deletions(-)
> > > 
> > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > index 82685dc..e6435da 100644
> > > --- a/target-i386/cpu.c
> > > +++ b/target-i386/cpu.c
> > > @@ -145,15 +145,17 @@ static uint32_t kvm_default_features = (1 << 
> > > KVM_FEATURE_CLOCKSOURCE) |
> > >          (1 << KVM_FEATURE_ASYNC_PF) |
> > >          (1 << KVM_FEATURE_STEAL_TIME) |
> > >          (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT);
> > > -static const uint32_t kvm_pv_eoi_features = (0x1 << KVM_FEATURE_PV_EOI);
> > >  #else
> > >  static uint32_t kvm_default_features = 0;
> > > -static const uint32_t kvm_pv_eoi_features = 0;
> > >  #endif
> > >  
> > >  void enable_kvm_pv_eoi(void)
> > >  {
> > > -    kvm_default_features |= kvm_pv_eoi_features;
> > > +#ifdef CONFIG_KVM
> > You do not need ifdef here.
> 
> We need it because KVM_FEATURE_PV_EOI is available only if CONFIG_KVM is
> set.
> 
> I could also write it as:
> 
>     if (kvm_enabled()) {
> #ifdef CONFIG_KVM
>         kvm_default_features |= (1UL << KVM_FEATURE_PV_EOI);
> #endif
>     }
> 
> But I find it less readable.
> 
> 
Why not define KVM_FEATURE_PV_EOI unconditionally?

> > 
> > > +    if (kvm_enabled()) {
> > > +        kvm_default_features |= (1UL << KVM_FEATURE_PV_EOI);
> > > +    }
> > > +#endif
> > >  }
> > >  
> > >  void host_cpuid(uint32_t function, uint32_t count,
> > > -- 
> > > 1.7.11.7
> > 
> > --
> >                     Gleb.
> 
> -- 
> Eduardo

--
                        Gleb.