Re: [PATCH v2 39/49] KVM: x86: Extract code for generating per-entry emulated CPUID information

[Sean Christopherson]

On Thu, Jul 04, 2024, Maxim Levitsky wrote:
> On Fri, 2024-05-17 at 10:39 -0700, Sean Christopherson wrote:
> PS: I spoke with Paolo about the meaning of KVM_GET_EMULATED_CPUID, because
> it is not clear from the documentation what it does, or what it supposed to
> do because qemu doesn't use this IOCTL.
> 
> So this ioctl is meant to return a static list of CPU features which *can* be
> emulated by KVM, if the cpu doesn't support them, but there is a cost to it,
> so they should not be enabled by default.
> 
> This means that if you run 'qemu -cpu host', these features (like rdpid) will
> only be enabled if supported by the host cpu, however if you explicitly ask
> qemu for such a feature, like 'qemu -cpu host,+rdpid', qemu should not warn
> if the feature is not supported on host cpu but can be emulated (because kvm
> can emulate the feature, which is stated by KVM_GET_EMULATED_CPUID ioctl).
> 
> Qemu currently doesn't support this but the support can be added.
> 
> So I think that the two ioctls should be redefined as such:
> 
> KVM_GET_SUPPORTED_CPUID - returns all CPU features that are supported by KVM,
> supported by host hardware, or that KVM can efficiently emulate.
> 
> 
> KVM_GET_EMULATED_CPUID - returns all CPU features that KVM *can* emulate if
> the host cpu lacks support, but emulation is not efficient and thus these
> features should be used with care when not supported by the host (e.g only
> when the user explicitly asks for them).

Yep, that aligns with how I view the ioctls (I haven't read the documentaion,
mainly because I have a terrible habit of never reading docs).

> I can post a patch to fix this or you can add something like that to your
> patch series if you prefer.

Go ahead and post a patch, assuming it's just a documentation update.