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

Eduardo Habkost
2013-01-07T11:42:36Z

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.

>
> > + 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