Re: [PATCH v7 5/9] i386: expand Hyper-V features during CPU feature expansion time

[Vitaly Kuznetsov <vkuznets@redhat.com>]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Thu, Jun 03, 2021 at 01:48:31PM +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>
>> ---
>>  target/i386/cpu.c          |  4 ++++
>>  target/i386/kvm/kvm-stub.c |  5 +++++
>>  target/i386/kvm/kvm.c      | 24 ++++++++++++++++++++----
>>  target/i386/kvm/kvm_i386.h |  1 +
>>  4 files changed, 30 insertions(+), 4 deletions(-)
>> 
>> diff --git a/target/i386/cpu.c b/target/i386/cpu.c
>> index b2d8e5713911..159b7aa8f073 100644
>> --- a/target/i386/cpu.c
>> +++ b/target/i386/cpu.c
>> @@ -5990,6 +5990,10 @@ 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..f6e7e4466e1a 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;
>>  }
>> +
>> +bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp)
>> +{
>> +    abort();
>> +}
>> diff --git a/target/i386/kvm/kvm.c b/target/i386/kvm/kvm.c
>> index a5f8553af921..650fdd970d6e 100644
>> --- a/target/i386/kvm/kvm.c
>> +++ b/target/i386/kvm/kvm.c
>> @@ -1217,13 +1217,22 @@ 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 bool hyperv_expand_features(CPUState *cs, Error **errp)
>> +bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp)
>>  {
>> -    X86CPU *cpu = X86_CPU(cs);
>> +    CPUState *cs = CPU(cpu);
>>  
>>      if (!hyperv_enabled(cpu))
>>          return true;
>>  
>> +    /*
>> +     * 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 true;
>> +
>>      if (cpu->hyperv_passthrough) {
>>          cpu->hyperv_vendor_id[0] =
>>              hv_cpuid_get_host(cs, HV_CPUID_VENDOR_AND_MAX_FUNCTIONS, R_EBX);
>> @@ -1565,8 +1574,15 @@ int kvm_arch_init_vcpu(CPUState *cs)
>>  
>>      env->apic_bus_freq = KVM_APIC_BUS_FREQUENCY;
>>  
>> -    /* Paravirtualization CPUIDs */
>> -    if (!hyperv_expand_features(cs, &local_err)) {
>> +    /*
>> +     * kvm_hyperv_expand_features() is called here for the second time in case
>> +     * KVM_CAP_SYS_HYPERV_CPUID is not supported. While we can't possibly handle
>> +     * 'query-cpu-model-expansion' in this case as we don't have a KVM vCPU to
>> +     * check which Hyper-V enlightenments are supported and which are not, we
>> +     * can still proceed and check/expand Hyper-V enlightenments here so legacy
>> +     * behavior is preserved.
>> +     */
>
> Issue we might need to solve later: how can management software
> differentiate "Hyper-V enlightenments are not supported at all"
> from "we can't tell you which Hyper-V enlightenments are
> available because KVM_CAP_SYS_HYPERV_CPUID is unavailable"?
>

In case we're not considering *very* old kernels, basic Hyper-V
enlightenments should always be available so if
'query-cpu-model-expansion' doesn't give you anything (like now,
pre-patch) then you treat this as 'QEMU can't tell me what's supported'
and not like 'no Hyper-V enlightenments at all'.

>
>> +    if (!kvm_hyperv_expand_features(cpu, &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..54667b35f09c 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);
>> +bool kvm_hyperv_expand_features(X86CPU *cpu, Error **errp);
>>  
>>  uint64_t kvm_swizzle_msi_ext_dest_id(uint64_t address);
>
> Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>

Thanks!

-- 
Vitaly