Re: [Qemu-devel] [PATCH 5/5] target-i386: fix kvm_default_features overwriting kvm_arch_get_supported_cpuid()

[Eduardo Habkost]

On Tue, Feb 05, 2013 at 07:46:09PM +0100, Igor Mammedov wrote:
> On Tue, 5 Feb 2013 15:53:04 -0200
> Eduardo Habkost <address@hidden> wrote:
> 
> > On Tue, Feb 05, 2013 at 05:39:24PM +0100, Igor Mammedov wrote:
> > > ORing kvm_default_features to def->kvm_features might set unsupported
> > > bits if kvm_arch_get_supported_cpuid() returns less bits set than in
> > > kvm_default_features. Fix it by removing kvm_default_features and using
> > > only what kvm_arch_get_supported_cpuid() returns.
> > 
> > This is exactly what we _must not_ do! We can't change CPUID bits in a
> > machine-type or we will change the guest ABI. We must keep them stable
> > for a machine-type and:
> > 
> > * Not change if QEMU is upgraded;
> > * Not change if the host kernel is upgraded;
> > * Not change if running on different host hardware.
> Is all of this applicable to CPU 'host' or only to built-in cpu_models?

This applies only to the built-in CPU models and the stable (versioned)
machine-types.

-cpu host is special and is expected to break the rules above. We could
also create a "pc-next" machine-type where those rules could be broken.


> 
> > 
> > The only way we are allowed to change the guest ABI is on a new
> > machine-type. This was the whole point of the bug fix at commit
> > ef8621b1a3, and this patch reintroduces the bug.
> > 
> > If you are starting QEMU in a host that doesn't support the set of KVM
> > features QEMU is configured to expose (through the command-line or by
> > choosing a machine-type), it _should_ fail to start if using -cpu
> > enforce. This is a feature.
> > 
> > We could enable this behavior on the "pc-next" machine, if we create it
> > one day, but not on the stable machine-types.
> > 
> > 
> > > 
> > > Mark disable_kvm_pv_eoi() to be removed in favor of compat_props
> > > when available.
> > > 
> > > Signed-off-by: Igor Mammedov <address@hidden>
> > > ---
> > >  target-i386/cpu.c |   20 ++++++++------------
> > >  target-i386/cpu.h |    2 ++
> > >  2 files changed, 10 insertions(+), 12 deletions(-)
> > > 
> > > diff --git a/target-i386/cpu.c b/target-i386/cpu.c
> > > index 62fdc84..e66362e 100644
> > > --- a/target-i386/cpu.c
> > > +++ b/target-i386/cpu.c
> > > @@ -220,17 +220,12 @@ typedef struct model_features_t {
> > >  int check_cpuid = 0;
> > >  int enforce_cpuid = 0;
> > >  
> > > -static uint32_t kvm_default_features = (1 << KVM_FEATURE_CLOCKSOURCE) |
> > > -        (1 << KVM_FEATURE_NOP_IO_DELAY) |
> > > -        (1 << KVM_FEATURE_CLOCKSOURCE2) |
> > > -        (1 << KVM_FEATURE_ASYNC_PF) |
> > > -        (1 << KVM_FEATURE_STEAL_TIME) |
> > > -        (1 << KVM_FEATURE_PV_EOI) |
> > > -        (1 << KVM_FEATURE_CLOCKSOURCE_STABLE_BIT);
> > > -
> > > +/* TODO: replace it with compat properties when feature bits
> > > + * are converted into static properties */
> > > +bool x86_cpu_no_pv_eoi;
> > >  void disable_kvm_pv_eoi(void)
> > >  {
> > > -    kvm_default_features &= ~(1UL << KVM_FEATURE_PV_EOI);
> > > +    x86_cpu_no_pv_eoi = true;
> > >  }
> > >  
> > >  void host_cpuid(uint32_t function, uint32_t count,
> > > @@ -2061,9 +2056,6 @@ static void x86_cpu_initfn(Object *obj)
> > >      env->cpuid_ext3_features = def->ext3_features;
> > >      object_property_set_int(OBJECT(cpu), def->xlevel, "xlevel", NULL);
> > >      env->cpuid_kvm_features = def->kvm_features;
> > > -    if (kvm_enabled()) {
> > > -        env->cpuid_kvm_features |= kvm_default_features;
> > > -    }
> > >      env->cpuid_svm_features = def->svm_features;
> > >      env->cpuid_ext4_features = def->ext4_features;
> > >      env->cpuid_7_0_ebx_features = def->cpuid_7_0_ebx_features;
> > > @@ -2107,6 +2099,10 @@ static void x86_cpu_def_class_init(ObjectClass 
> > > *oc, void *data)
> > >           */
> > >          memcpy(xcc->info.vendor, hostcc->info.vendor,
> > >                 sizeof(xcc->info.vendor));
> > > +        xcc->info.kvm_features = xcc->info.kvm_features;
> > > +        if (x86_cpu_no_pv_eoi) {
> > > +            xcc->info.kvm_features &= ~(1UL << KVM_FEATURE_PV_EOI);
> > > +        }
> > >      }
> > >  
> > >      /* Look for specific models that have the QEMU version in .model_id 
> > > */
> > > diff --git a/target-i386/cpu.h b/target-i386/cpu.h
> > > index 11ef942..51f20b6 100644
> > > --- a/target-i386/cpu.h
> > > +++ b/target-i386/cpu.h
> > > @@ -1246,6 +1246,8 @@ void do_smm_enter(CPUX86State *env1);
> > >  
> > >  void cpu_report_tpr_access(CPUX86State *env, TPRAccess access);
> > >  
> > > +/* TODO: convert to compat_props */
> > > +extern bool x86_cpu_no_pv_eoi;
> > >  void disable_kvm_pv_eoi(void);
> > >  
> > >  /* Return name of 32-bit register, from a R_* constant */
> > > -- 
> > > 1.7.1
> > > 
> > 
> > -- 
> > Eduardo
> 
> 
> -- 
> Regards,
>   Igor

-- 
Eduardo