Re: [PATCH RFC 12/22] i386: always fill Hyper-V CPUID feature leaves from X86CPU data

[Vitaly Kuznetsov]

Eduardo Habkost <ehabkost@redhat.com> writes:

> On Fri, Sep 04, 2020 at 04:54:21PM +0200, Vitaly Kuznetsov wrote:
>> We have all the required data in X86CPU already and as we are about to
>> split hyperv_handle_properties() into hyperv_expand_features()/
>> hyperv_fill_cpuids() we can remove the blind copy. The functional change
>> is that QEMU won't pass CPUID leaves it doesn't currently know about
>> to the guest but arguably this is a good change.
>
> Is it, though?  Maybe we don't want it to be the default, but a
> blind passthrough mode that copies all CPUID leaves would be
> useful for testing new features.
>

We already have Hyper-V features which require extra work to get enabled
(SynIC, EVMCS). Passing corresponding feature bits to guests without
actually enabling these features in KVM will likely break them. So a pure
'blind passthrough' mode is not possible. 

>> 
>> Signed-off-by: Vitaly Kuznetsov <vkuznets@redhat.com>
>> ---
>>  target/i386/kvm.c | 9 ---------
>>  1 file changed, 9 deletions(-)
>> 
>> diff --git a/target/i386/kvm.c b/target/i386/kvm.c
>> index 87b83a2aa2cb..b7f0e200a75f 100644
>> --- a/target/i386/kvm.c
>> +++ b/target/i386/kvm.c
>> @@ -1221,9 +1221,6 @@ static int hyperv_handle_properties(CPUState *cs,
>>      }
>>  
>>      if (cpu->hyperv_passthrough) {
>> -        memcpy(cpuid_ent, &cpuid->entries[0],
>> -               cpuid->nent * sizeof(cpuid->entries[0]));
>> -
>>          c = cpuid_find_entry(cpuid, HV_CPUID_VENDOR_AND_MAX_FUNCTIONS, 0);
>>          if (c) {
>>              cpu->hyperv_vendor_id[0] = c->ebx;
>> @@ -1325,12 +1322,6 @@ static int hyperv_handle_properties(CPUState *cs,
>>          goto free;
>>      }
>>  
>> -    if (cpu->hyperv_passthrough) {
>> -        /* We already copied all feature words from KVM as is */
>> -        r = cpuid->nent;
>> -        goto free;
>> -    }
>> -
>>      c = &cpuid_ent[cpuid_i++];
>>      c->function = HV_CPUID_VENDOR_AND_MAX_FUNCTIONS;
>>      c->eax = hyperv_feat_enabled(cpu, HYPERV_FEAT_EVMCS) ?
>> -- 
>> 2.25.4
>> 

-- 
Vitaly