2016-07-01 17:12+0200, Paolo Bonzini: > On 01/07/2016 17:06, Paolo Bonzini wrote: >>>> >> > Should it? >>> Yes, x2APIC ID cannot be changed in hardware and is initialized to the >>> intitial APIC ID. >>> Letting LAPIC_SET change x2APIC ID would allow scenarios where userspace >>> reuses old VMs instead of building new ones after reconfiguration. >>> I don't think it's a sensible use case and it it is currently broken, >>> because we don't exit to userspace when changing APIC mode, so KVM would >>> just set APIC ID to VCPU ID on any transition and userspace couldn't >>> amend it. > > Forgot to reply about this: letting SET_LAPIC change x2APIC IDs is nonsense. > > In x2APIC mode + new capability disabled SET_LAPIC should ignore the id > register altogether for backwards compatibility.
I'd still shift SET_LAPIC APIC ID to have internal APIC ID register in hardware-compatible format. > In x2APIC mode + new capability enabled it should either ignore it, or > fail if the x2APIC ID doesn't match the VCPU id. I suspect the latter > is better because it would help catching the case where userspace is > erroneously shifting the id left to bits 31-24. Yes, I'll make it EINVAL.
