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

Gleb Natapov
2013-01-07T11:42:53Z

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?

> >
> > > + 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.