Re: [PATCH v4 17/21] i386: support 'hv-passthrough, hv-feature=off' on the command line

[Igor Mammedov <imammedo@redhat.com>]

On Fri, 12 Feb 2021 09:49:46 +0100
Vitaly Kuznetsov <vkuznets@redhat.com> wrote:

> Igor Mammedov <imammedo@redhat.com> writes:
> 
> > On Wed, 10 Feb 2021 17:40:29 +0100
> > Vitaly Kuznetsov <vkuznets@redhat.com> wrote:
> >  
> >> Currently, we support 'hv-passthrough,hv-feature=on' enablement, this
> >> is supposed to mean "hv-feature is mandatory, don't start without it". Add
> >> support for 'hv-passthrough,hv-feature=off' meaning "enable everything
> >> supported by the host except for hv-feature".
> >> 
> >> While on it, make 'hv-passthrough' parse semantics in-line with other
> >> options in qemu: when specified, it overrides what was previously set with
> >> what's supported by the host. This can later be modified with 'hv-feature=on'/
> >> 'hv-feature=off'.
> >> 
> >> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
> >> ---
> >>  target/i386/cpu.c     | 28 +++++++++++++++++++++++++++-
> >>  target/i386/kvm/kvm.c |  4 ++++
> >>  2 files changed, 31 insertions(+), 1 deletion(-)
> >> 
> >> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> >> index e8a004c39d04..f8df2caed779 100644
> >> --- a/target/i386/cpu.c
> >> +++ b/target/i386/cpu.c
> >> @@ -4725,6 +4725,29 @@ static void x86_hv_stimer_direct_set(Object *obj, bool value, Error **errp)
> >>      x86_hv_feature_set(obj, value, HYPERV_FEAT_STIMER_DIRECT);
> >>  }
> >>  
> >> +static bool x86_hv_passthrough_get(Object *obj, Error **errp)
> >> +{
> >> +    X86CPU *cpu = X86_CPU(obj);
> >> +
> >> +    return cpu->hyperv_passthrough;
> >> +}
> >> +
> >> +static void x86_hv_passthrough_set(Object *obj, bool value, Error **errp)
> >> +{
> >> +    X86CPU *cpu = X86_CPU(obj);
> >> +
> >> +    cpu->hyperv_passthrough = value;
> >> +
> >> +    /* hv-passthrough overrides everything with what's supported by the host */
> >> +    if (value) {
> >> +        cpu->hyperv_features = 0;
> >> +        cpu->hyperv_features_on = 0;
> >> +        cpu->hyperv_features_off = 0;  
> >
> > why do we have _on|_off fields?
> >  
> 
> You mean 'why do we have them at all' or 'why do we reset them here'?
> For the former, we need to distinguish between
> 'hv-passthroug,hv-feature=off' and just 'hv-passthrough';
> 'hv-passthrough,hv-evmcs=on' and just 'hv-passthrough'. For the later,

that's what I was asking for, eventually I found it being buried in kvm code

> I'd like to make the samentics you've asked for:
> 'hv-feature,hv-passthrough' == 'hv-passthrough'
you essentially you wrote your own hv-foo parser in kvm_hyperv_expand_features(),
which is a bit complicated for my taste.
With scratch CPU you can simplify and make it easier to read.

> (though I still see it as a gotcha for an unprepared user)
this way it at least works the same way like any other property.

 
> >> +    }
> >> +
> >> +    return;
> >> +}
> >> +
> >>  /* Generic getter for "feature-words" and "filtered-features" properties */
> >>  static void x86_cpu_get_feature_words(Object *obj, Visitor *v,
> >>                                        const char *name, void *opaque,
> >> @@ -7281,7 +7304,6 @@ static Property x86_cpu_properties[] = {
> >>                         HYPERV_SPINLOCK_NEVER_NOTIFY),
> >>      DEFINE_PROP_ON_OFF_AUTO("hv-no-nonarch-coresharing", X86CPU,
> >>                              hyperv_no_nonarch_cs, ON_OFF_AUTO_OFF),
> >> -    DEFINE_PROP_BOOL("hv-passthrough", X86CPU, hyperv_passthrough, false),
> >>  
> >>      DEFINE_PROP_BOOL("check", X86CPU, check_cpuid, true),
> >>      DEFINE_PROP_BOOL("enforce", X86CPU, enforce_cpuid, false),
> >> @@ -7460,6 +7482,10 @@ static void x86_cpu_common_class_init(ObjectClass *oc, void *data)
> >>                                     x86_hv_stimer_direct_get,
> >>                                     x86_hv_stimer_direct_set);
> >>  
> >> +    object_class_property_add_bool(oc, "hv-passthrough",
> >> +                                   x86_hv_passthrough_get,
> >> +                                   x86_hv_passthrough_set);
> >> +
> >>      for (w = 0; w < FEATURE_WORDS; w++) {
> >>          int bitnr;
> >>          for (bitnr = 0; bitnr < 64; bitnr++) {
> >> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
> >> index 30013f0d7cee..fca088d4d3b5 100644
> >> --- a/target/i386/kvm/kvm.c
> >> +++ b/target/i386/kvm/kvm.c
> >> @@ -1153,6 +1153,10 @@ static int hv_cpuid_check_and_set(CPUState *cs, int feature, Error **errp)
> >>          return 0;
> >>      }
> >>  
> >> +    if (cpu->hyperv_passthrough && (cpu->hyperv_features_off & BIT(feature))) {
> >> +        return 0;
> >> +    }
> >> +
> >>      deps = kvm_hyperv_properties[feature].dependencies;
> >>      while (deps) {
> >>          dep_feat = ctz64(deps);  
> >  
>