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



On Mon, Jan 07, 2013 at 01:42:53PM +0200, Gleb Natapov wrote:
> 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 <ehabkost redhat com>
> > > > ---
> > > > Cc: kvm vger kernel org
> > > > Cc: Michael S. Tsirkin <mst redhat com>
> > > > Cc: Gleb Natapov <gleb redhat com>
> > > > Cc: Marcelo Tosatti <mtosatti redhat com>
> > > > 
> > > > 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?

It comes from the KVM kernel headers, that are included only if
CONFIG_KVM is set, and probably won't even compile in non-Linux systems.

I have a dejavu feeling. I believe we had this exact problem before,
maybe about some other #defines that come from the Linux KVM headers and
won't be available in non-Linux systems.

> 
> > > 
> > > > +    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.

-- 
Eduardo


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