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

[Eduardo Habkost]

On Wed, Jan 02, 2013 at 09:30:20PM +0100, Igor Mammedov wrote:
> On Wed, 2 Jan 2013 13:29:10 -0200
> Eduardo Habkost <address@hidden> 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 <address@hidden> 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.

Just like you may end up getting new features enabled by -cpu host after
upgrading the kernel, you may end up getting new features enabled by
-cpu host after upgrading qemu. If you don't like surprises, don't use
-cpu host.  ;-)

I don't think machine-types exist to avoid user surprise, they exist to
keep compatibility (which is not expected to happen when using -cpu
host). Keeping compatibility is hard enough in the cases where we really
need it, I don't think it is worth the extra work and complexity for an
use case where compatibility is not expected.

> 
> > 
> > > 
> > > > 
> > > > 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.

The best way I see to simplify kvm_check_features_against_host()
requires the feature words array patch, that touches _lots_ of code. I
wanted to avoid adding such an intrusive patch as a dependency.

> 
> > 
> > > 
> > > > 
> > > > Signed-off-by: Eduardo Habkost <address@hidden>
> > > > ---
> > > >  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

-- 
Eduardo