Re: [Qemu-devel] [PATCH v4 02/17] target-i386: Add missing kvm bits.

[Eduardo Habkost]

On Fri, Sep 21, 2012 at 10:39:52AM +0200, Igor Mammedov wrote:
> On Thu, 20 Sep 2012 16:03:17 -0400
> Don Slutz <address@hidden> wrote:
> 
> > Fix duplicate name (kvmclock => kvm_clock2) also.
> > 
> > Signed-off-by: Don Slutz <address@hidden>
> > ---
> >  target-i386/cpu.c |   12 ++++++++----
> >  1 files changed, 8 insertions(+), 4 deletions(-)
> > 
> > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > index 0313cf5..5f9866a 100644
> > --- a/target-i386/cpu.c
> > +++ b/target-i386/cpu.c
> > @@ -87,10 +87,14 @@ static const char *ext3_feature_name[] = {
> >  };
> >  
> >  static const char *kvm_feature_name[] = {
> > -    "kvmclock", "kvm_nopiodelay", "kvm_mmu", "kvmclock", "kvm_asyncpf", 
> > NULL, "kvm_pv_eoi", NULL,
> > -    NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
> > -    NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
> > -    NULL, NULL, NULL, NULL, NULL, NULL, NULL, NULL,
> > +    "kvmclock", "kvm_nopiodelay", "kvm_mmu", "kvm_clock2",
> before patch if "kvmclock" is specified it would set 0 and 3 bits,
> after patch only bit 0 is set.
> Is it correct/expected behavior? if yes, please add rationale into patch
> description.

The problem here seems to be:
- It would be interesting to make "kvmclock=true" enough to enable the
  optimal behavior, instead of requiring users to use "kvm_clock2=true"
  explicitly
- We need to allow older machine-types to be backwards compatible (not
  enabling the second bit by default), so we need a separate property
  to control the second bit.

I think this is best modelled this way:

- Having two separate properties: kvmclock and kvmclock2 (or kvm_clock2)
- Older machine-types would have kvmclock2 default to false. Newer
  machine-types would kvmclock2 default to true.
- kvmclock=false would disable both bits

Then:

 - kvmclock=false would not set any bit (it would be surprising to have
   kvmclock=false but still have kvmclock enabled)
 - kvmclock=true would keep compatible behavior on older machine-types,
   (only the first bit set), but would get optimal behavior on newer
   machine-types (both bits set)
 - kvmclock=true,kvmclock2=true would set both bits
 - kvmclock=true,kvmclock2=false would set only the first bit

It wouldn't be a direct mapping between properties and CPUID bits, but
that's exactly the point. In this case, exposing individual CPUID bits
directly is a too low-level interface.


>  
> > +    "kvm_asyncpf", "kvm_steal_time", "kvm_pv_eoi", NULL,
> > +    NULL, NULL, NULL, NULL,
> > +    NULL, NULL, NULL, NULL,
> > +    NULL, NULL, NULL, NULL,
> > +    NULL, NULL, NULL, NULL,
> > +    "kvm_clock_stable", NULL, NULL, NULL,
> > +    NULL, NULL, NULL, NULL,
> >  };
> >  
> >  static const char *svm_feature_name[] = {
> > -- 
> > 1.7.1
> > 
> 
> 
> -- 
> Regards,
>   Igor

-- 
Eduardo