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.
