Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic_map

[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.

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.

Paolo