Re: [PATCH v6 15/19] i386: expand Hyper-V features during CPU feature expansion time

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Thu, Apr 22, 2021 at 06:11:26PM +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>
>> ---
>>  target/i386/cpu.c          |  4 ++++
>>  target/i386/kvm/kvm-stub.c |  5 +++++
>>  target/i386/kvm/kvm.c      | 15 ++++++++++++---
>>  target/i386/kvm/kvm_i386.h |  1 +
>>  4 files changed, 22 insertions(+), 3 deletions(-)
>> 
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index d72b8760e7a3..18b57f3d8b9c 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -6514,6 +6514,10 @@ static void x86_cpu_expand_features(X86CPU *cpu, Error **errp)
>>      if (env->cpuid_xlevel2 == UINT32_MAX) {
>>          env->cpuid_xlevel2 = env->cpuid_min_xlevel2;
>>      }
>> +
>> +    if (kvm_enabled()) {
>> +        kvm_hyperv_expand_features(cpu, errp);
>> +    }
>>  }
>>  
>>  /*
>> diff --git a/target/i386/kvm/kvm-stub.c b/target/i386/kvm/kvm-stub.c
>> index 92f49121b8fa..7f175faa3abd 100644
>> --- a/target/i386/kvm/kvm-stub.c
>> +++ b/target/i386/kvm/kvm-stub.c
>> @@ -39,3 +39,8 @@ bool kvm_hv_vpindex_settable(void)
>>  {
>>      return false;
>>  }
>> +
>> +void kvm_hyperv_expand_features(X86CPU *cpu, Error **errp)
>> +{
>> +    return;
>
> Maybe we should make it abort()?  This function should never be
> called if KVM is disabled.
>

Sure, can do that.

>> +}
>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>> index a42263b24fca..d5551c4ab5cf 100644
>> --- a/target/i386/kvm/kvm.c
>> +++ b/target/i386/kvm/kvm.c
>> @@ -1216,13 +1216,22 @@ static uint32_t hv_build_cpuid_leaf(CPUState *cs, uint32_t func, int reg)
>>   * of 'hv_passthrough' mode and fills the environment with all supported
>>   * Hyper-V features.
>>   */
>> -static void hyperv_expand_features(CPUState *cs, Error **errp)
>> +void kvm_hyperv_expand_features(X86CPU *cpu, Error **errp)
>>  {
>> -    X86CPU *cpu = X86_CPU(cs);
>> +    CPUState *cs = CPU(cpu);
>>  
>>      if (!hyperv_enabled(cpu))
>>          return;
>>  
>> +    /*
>> +     * When kvm_hyperv_expand_features is called at CPU feature expansion
>> +     * time per-CPU kvm_state is not available yet so we can only proceed
>> +     * when KVM_CAP_SYS_HYPERV_CPUID is supported.
>> +     */
>> +    if (!cs->kvm_state &&
>> +        !kvm_check_extension(kvm_state, KVM_CAP_SYS_HYPERV_CPUID))
>> +        return;
>> +
>>      if (cpu->hyperv_passthrough) {
>>          cpu->hyperv_vendor_id[0] =
>>              hv_cpuid_get_host(cs, HV_CPUID_VENDOR_AND_MAX_FUNCTIONS, R_EBX);
>> @@ -1556,7 +1565,7 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>      env->apic_bus_freq = KVM_APIC_BUS_FREQUENCY;
>>  
>>      /* Paravirtualization CPUIDs */
>> -    hyperv_expand_features(cs, &local_err);
>> +    kvm_hyperv_expand_features(cpu, &local_err);
>
> Do we still need to call the function again here?
>
> If the first expansion isn't expanding everything, I'm afraid
> this second call will hide bugs in query-cpu-model-expansion.
>

The first expansion will do nothing if KVM_CAP_SYS_HYPERV_CPUID is not
supported, calling it here allows us to proceed. The series makes
'query-cpu-model-expansion' output correct only with
KVM_CAP_SYS_HYPERV_CPUID, without it we don't seem to be able to do much
(unless we decide to create a 'scratch' CPU or something like that).

>
>>      if (local_err) {
>>          error_report_err(local_err);
>>          return -ENOSYS;
>> diff --git a/target/i386/kvm/kvm_i386.h b/target/i386/kvm/kvm_i386.h
>> index dc725083891c..f1176491051d 100644
>> --- a/target/i386/kvm/kvm_i386.h
>> +++ b/target/i386/kvm/kvm_i386.h
>> @@ -47,6 +47,7 @@ bool kvm_has_x2apic_api(void);
>>  bool kvm_has_waitpkg(void);
>>  
>>  bool kvm_hv_vpindex_settable(void);
>> +void kvm_hyperv_expand_features(X86CPU *cpu, Error **errp);
>>  
>>  uint64_t kvm_swizzle_msi_ext_dest_id(uint64_t address);
>>  
>> -- 
>> 2.30.2
>> 

-- 
Vitaly