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

[Gleb Natapov <gleb@redhat.com>]

On Mon, Jan 07, 2013 at 10:30:40AM -0200, Eduardo Habkost wrote:
> On Mon, Jan 07, 2013 at 02:15:59PM +0200, Gleb Natapov wrote:
> > On Mon, Jan 07, 2013 at 10:09:24AM -0200, Eduardo Habkost wrote:
> > > 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.
> > >
> > It is better to hide all KVM related differences somewhere in the
> > headers where no one sees them instead of sprinkle them all over the
> > code. We can put those defines in include/sysemu/kvm.h in !CONFIG_KVM
> > part. Or have one ifdef CONFIG_KVM at the beginning of the file and
> > define enable_kvm_pv_eoi() there and provide empty stub otherwise.
> 
> If we had an empty enable_kvm_pv_eoi() stub, we would need an #ifdef
> around the real implementation. I mean, I don't think this:
> 
>   #ifdef CONFIG_KVM
>   int enable_kvm_pv_eoi() {
>     [...]
>   }
>   #endif
> 
You already have #ifdef CONFIG_KVM just above enable_kvm_pv_eoi(). Put
everything KVM related there instead of adding #ifdef CONFIG_KVM all
over the file.

> is any better than this:
> 
>   int enable_kvm_pv_eoi() {
>   #ifdef CONFIG_KVM
>     [...]
>   #endif
>   }
> 
> So this is probably a good reason to duplicate the KVM_FEATURE_*
> #defines in the QEMU code, instead?
> 
Not even duplicate, they can be fake just to keep compiler happy.

--
			Gleb.