On Tue, Oct 25, 2022, Paolo Bonzini wrote: > On 10/25/22 18:46, Sean Christopherson wrote: > > On Sat, Oct 22, 2022, Paolo Bonzini wrote: > > > Several fields of CPUID.8000001EH (ExtendedApicId in EAX[31:0], > > > CoreId in EBX[7:0], NodeId in ECX[7:0]) vary on each processor, > > > and it is simply impossible to fit the right values in the > > > KVM_GET_SUPPORTED_CPUID API, in such a way that they can be > > > passed to KVM_SET_CPUID2. > > > > The same is true for 0xb and 0x1f, why delete 0x8000001e but keep those? I agree > > that KVM_GET_SUPPORTED_CPUID can't get this right, but KVM can at least be > > consistent with itself. > > 0xb and 0x1f are already special cased because EDX is set to the X2APIC id. > KVM knows how to do that unlike the NodeId and CoreId. But KVM doesn't properly support 0xB/0x1F. E.g. if usersepace regurgitates KVM_GET_SUPPORTED_CPUID back into KVM_SET_CPUID2, all vCPUs will observe the same x2APIC ID in EDX, and it will be a host x2APIC ID to boot. KVM only handles the where userspace provides 0xB.1 (or 0x1F.1), the guest performs CPUID with ECX>1, _and_ userspace doesn't provide the exact CPUID entry. I suppose one could argue that KVM needs to communicate to userspace that KVM emulates the edge case behavior of CPUID 0xB and 0x1F, but I would argue that KVM communicates that by announcing a max basic leaf >= 0xB/0x1F.
