Re: [PATCH RFC 22/22] i386: expand Hyper-V features early

[Vitaly Kuznetsov]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Fri, Sep 04, 2020 at 04:54:31PM +0200, Vitaly Kuznetsov wrote:
>> To make Hyper-V features appear in e.g. QMP query-cpu-model-expansion we
>> need to expand and set the corresponding CPUID leaves early. Modify
>> x86_cpu_get_supported_feature_word() to call newly intoduced Hyper-V
>> specific kvm_hv_get_supported_cpuid() instead of
>> kvm_arch_get_supported_cpuid(). We can't use kvm_arch_get_supported_cpuid()
>> as Hyper-V specific CPUID leaves intersect with KVM's.
>> 
>> Note, early expansion will only happen when KVM supports system wide
>> KVM_GET_SUPPORTED_HV_CPUID ioctl (KVM_CAP_SYS_HYPERV_CPUID).
>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
> [...]
>> +uint32_t kvm_hv_get_supported_cpuid(X86CPU *cpu, enum FeatureWord w)
>> +{
>> +    CPUState *cs = CPU(cpu);
>> +    CPUX86State *env = &cpu->env;
>> +    Error *local_err = NULL;
>> +
>> +    hyperv_expand_features(cs, &local_err);
>
> This makes a function that sounds like it doesn't have any side
> effect ("get supported cpuid") have an unintended side effect
> (hyperv_expand_features() will change all CPUID data inside the
> cpu object).
>
> What about making it more similar to
> kvm_arch_get_supported_cpuid(), and be just a wrapper to
> get_supported_hv_cpuid()?
>
> I would also make sure get_supported_hv_cpuid() doesn't get
> CPUState as argument, just to be sure it will never touch the CPU
> object state.

Sure, I can try. The difference with get_supported_cpuid() is that
get_supported_hv_cpuid() currently doesn't have cache and we don't
probably want to call KVM_GET_SUPPORTED_HV_CPUID ioctl for every CPUID
leaf. I don't see why it can't be introduced though.

>
>> +
>> +    if (local_err) {
>> +            error_report_err(local_err);
>> +    }
>> +
>> +    return env->features[w];
>> +}
>> +
>>  static Error *hv_passthrough_mig_blocker;
>>  static Error *hv_no_nonarch_cs_mig_blocker;
>>  
>> diff --git a/target/i386/kvm_i386.h b/target/i386/kvm_i386.h
>> index 064b8798a26c..2e7da4f39668 100644
>> --- a/target/i386/kvm_i386.h
>> +++ b/target/i386/kvm_i386.h
>> @@ -48,4 +48,11 @@ bool kvm_has_waitpkg(void);
>>  
>>  bool kvm_hv_vpindex_settable(void);
>>  
>> +static inline bool hyperv_feature_word(enum FeatureWord w)
>> +{
>> +    return w >= FEAT_HYPERV_EAX && w <= FEAT_HV_NESTED_EDX;
>> +}
>> +
>> +uint32_t kvm_hv_get_supported_cpuid(X86CPU *cpu, enum FeatureWord w);
>> +
>>  #endif
>> -- 
>> 2.25.4
>> 

-- 
Vitaly