Re: [Qemu-arm] [Qemu-devel] [PATCH v2 02/14] target/arm/cpu: Ensure we can use the pmu with kvm

[Andrew Jones]

On Wed, Jun 26, 2019 at 11:49:03AM +0200, Richard Henderson wrote:
> On 6/21/19 6:34 PM, Andrew Jones wrote:
> > We first convert the pmu property from a static property to one with
> > its own accessors. Then we use the set accessor to check if the PMU is
> > supported when using KVM. Indeed a 32-bit KVM host does not support
> > the PMU, so this check will catch an attempt to use it at property-set
> > time.
> > 
> > Signed-off-by: Andrew Jones <address@hidden>
> > ---
> >  target/arm/cpu.c     | 30 +++++++++++++++++++++++++-----
> >  target/arm/kvm.c     |  9 +++++++++
> >  target/arm/kvm_arm.h | 13 +++++++++++++
> >  3 files changed, 47 insertions(+), 5 deletions(-)
> > 
> > diff --git a/target/arm/cpu.c b/target/arm/cpu.c
> > index 376db154f008..858f668d226e 100644
> > --- a/target/arm/cpu.c
> > +++ b/target/arm/cpu.c
> > @@ -759,10 +759,6 @@ static Property arm_cpu_has_el3_property =
> >  static Property arm_cpu_cfgend_property =
> >              DEFINE_PROP_BOOL("cfgend", ARMCPU, cfgend, false);
> >  
> > -/* use property name "pmu" to match other archs and virt tools */
> > -static Property arm_cpu_has_pmu_property =
> > -            DEFINE_PROP_BOOL("pmu", ARMCPU, has_pmu, true);
> > -
> >  static Property arm_cpu_has_vfp_property =
> >              DEFINE_PROP_BOOL("vfp", ARMCPU, has_vfp, true);
> >  
> > @@ -785,6 +781,29 @@ static Property arm_cpu_pmsav7_dregion_property =
> >                                             pmsav7_dregion,
> >                                             qdev_prop_uint32, uint32_t);
> >  
> > +static bool arm_get_pmu(Object *obj, Error **errp)
> > +{
> > +    ARMCPU *cpu = ARM_CPU(obj);
> > +
> > +    return cpu->has_pmu;
> > +}
> > +
> > +static void arm_set_pmu(Object *obj, bool value, Error **errp)
> > +{
> > +    ARMCPU *cpu = ARM_CPU(obj);
> > +
> > +    if (value) {
> > +        if (kvm_enabled() && !kvm_arm_pmu_supported(CPU(cpu))) {
> > +            error_setg(errp, "'pmu' feature not supported by KVM on this 
> > host");
> > +            return;
> > +        }
> > +        set_feature(&cpu->env, ARM_FEATURE_PMU);
> > +    } else {
> > +        unset_feature(&cpu->env, ARM_FEATURE_PMU);
> > +    }
> > +    cpu->has_pmu = value;
> > +}
> > +
> >  static void arm_get_init_svtor(Object *obj, Visitor *v, const char *name,
> >                                 void *opaque, Error **errp)
> >  {
> > @@ -859,7 +878,8 @@ void arm_cpu_post_init(Object *obj)
> >      }
> >  
> >      if (arm_feature(&cpu->env, ARM_FEATURE_PMU)) {
> > -        qdev_property_add_static(DEVICE(obj), &arm_cpu_has_pmu_property,
> > +        cpu->has_pmu = true;
> > +        object_property_add_bool(obj, "pmu", arm_get_pmu, arm_set_pmu,
> >                                   &error_abort);
> 
> This doesn't look right.
> 
> The static property is only enabled here if the cpu is known to support the
> PMU, and thus the only useful setting is -cpu foo,pmu=off.  Which means that
> the extra checking that you do in the dynamic property is unused.

It's used when attempting to do '-cpu foo,pmu=on' on a cpu model that
would support the PMU on TCG, and thus has the property, but won't work
when KVM is in use. I'll admit I didn't test this, because I don't have
32-bit KVM hosts available, but I'm pretty sure it should work as
expected:

 -accel tcg -cpu foo                <-- on by default
 -accel tcg -cpu foo,pmu=off        <-- off
 -accel tcg -cpu foo,pmu=on         <-- nop

 -accel kvm -cpu foo                <-- on by default if kvm+foo supports
                                        the pmu, otherwise off
 -accel kvm -cpu foo,pmu=off        <-- off
 -accel kvm -cpu foo,pmu=on         <-- nop if kvm+foo supports the pmu,
                                        otherwise error message

With the error message, which will only occur with 32-bit kvm hosts, since
64-bit kvm hosts have all supported the pmu for quite some time, being the
only new thing.

> 
> It seems like you need to be checking for the PMU earlier, e.g. in
> kvm_arm_get_host_cpu_features.

We need to push all the checks into property accessors if we want the
QMP command to work for them.

Thanks,
drew