Re: [Qemu-devel] [PATCH 2/2] target-i386: kvm: enable all supported KVM features for -cpu host

[Igor Mammedov <imammedo@redhat.com>]

On Wed, 2 Jan 2013 13:29:10 -0200
Eduardo Habkost <ehabkost@redhat.com> wrote:

> On Wed, Jan 02, 2013 at 03:52:45PM +0100, Igor Mammedov wrote:
> > On Fri, 28 Dec 2012 16:37:34 -0200
> > Eduardo Habkost <ehabkost@redhat.com> wrote:
> > 
> > > When using -cpu host, we don't need to use the kvm_default_features
> > > variable, as the user is explicitly asking QEMU to enable all feature
> > > supported by the host.
> > > 
> > > This changes the kvm_cpu_fill_host() code to use GET_SUPPORTED_CPUID to
> > > initialize the kvm_features field, so we get all host KVM features
> > > enabled.
> > 
> > 1_2 and 1_3 compat machines diff on pv_eoi flag, with this patch 1_2 might
> > have it set.
> > Is it ok from compat machines pov?
> 
> -cpu host is completely dependent on host hardware and kernel version,
> there are no compatibility expectations.

It's still kind of unpleasant surprise if on the same host
"qemu-1.3 -cpu host -machine pc-1.2" and "qemu-1.3+ -cpu host -machine pc-1.2"
would give different pv_eoi feature default, where pv-eoi should be
available after -machine pc-1.2 by default.

> 
> > 
> > > 
> > > This will also allow use to properly check/enforce KVM features inside
> > > kvm_check_features_against_host() later. For example, we will be able to
> > > make this:
> > > 
> > >   $ qemu-system-x86_64 -cpu ...,+kvm_pv_eoi,enforce
> > > 
> > > refuse to start if kvm_pv_eoi is not supported by the host (after we fix
> > > kvm_check_features_against_host() to check KVM flags as well).
> > It would be nice to have kvm_check_features_against_host() patch in this
> > series to verify that this patch and previous patch works as expected.
> 
> The kvm_check_features_against_host() change would be a user-visible
> behavior change, and I wanted to keep the changes minimal by now. (the
> main reason I submitted this earlier is to make it easier to clean up
> the init code for CPU subclasses)
> 
> I was planning to introduce those behavior changes only after
> introducing the feature-word array, so the kvm_check_features_against_host()
> code can become simpler and easier to review (instead of adding 4
> additional items to the messy struct model_features_t array). But if you
> think we can introduce those changes now, I will be happy to send a
> series that changes that code as well.
It would be better if it and simplifying kvm_check_features_against_host()
were in here together.

> 
> > 
> > > 
> > > Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
> > > ---
> > >  target-i386/cpu.c | 2 ++
> > >  1 file changed, 2 insertions(+)
> > > 
> > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > index 6e2d32d..76f19f0 100644
> > > --- a/target-i386/cpu.c
> > > +++ b/target-i386/cpu.c
> > > @@ -900,6 +900,8 @@ static void kvm_cpu_fill_host(x86_def_t *x86_cpu_def)
> > >      /* Other KVM-specific feature fields: */
> > >      x86_cpu_def->svm_features =
> > >                  kvm_arch_get_supported_cpuid(s, 0x8000000A, 0, R_EDX);
> > > +    x86_cpu_def->kvm_features =
> > > +                kvm_arch_get_supported_cpuid(s, KVM_CPUID_FEATURES, 0,
> > > R_EAX); 
> > >  #endif /* CONFIG_KVM */
> > >  }
> > 
> 
> -- 
> Eduardo
> 


-- 
Regards,
  Igor