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

Radim Krčmář
2016-07-01T15:51:55Z

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.