Re: [PATCH v6 2/5] KVM: x86: Add IBPB support

[Paolo Bonzini]

On 02/02/2018 21:28, Konrad Rzeszutek Wilk wrote:
>>>> Nothing would *set* the IBPB bit though, since that's a "virtual" bit
>>>> on Intel hardware. The comment explains why we have that |= F(IBPB),
>>>> and if the comment wasn't true, we wouldn't need that code either.
>>> But this seems wrong. That is on Intel CPUs we will advertise on
>>> AMD leaf that the IBPB feature is available.
>>>
>>> Shouldn't we just check to see if the machine is AMD before advertising
>>> this bit?
>> No. The AMD feature bits give us more fine-grained support for exposing
>> IBPB or IBRS alone, so we expose those bits on Intel too.
> But but.. that runs smack against the idea of exposing a platform that
> is as close to emulating the real hardware as possible.
> 
> As in I would never expect an Intel CPU to expose the IBPB on the 0x8000_0008
> leaf. Hence KVM (nor any hypervisor) should not do it either.

This is KVM_GET_*SUPPORTED*_CPUID.  The actual CPUID bits that are
exposed (and also which CPUID leafs are there, even though this one is
present in both Intel and AMD) are determined by userspace.

Paolo