linux-kernel - Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic

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Message-ID: <46f7b2e0-3225-8a9b-6155-3588b5f60f78@redhat.com>
Date:	Fri, 1 Jul 2016 17:12:15 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Radim Krčmář <rkrcmar@...hat.com>
Cc:	Andrew Honig <ahonig@...gle.com>, linux-kernel@...r.kernel.org,
	kvm <kvm@...r.kernel.org>, "Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>
Subject: Re: [PATCH v1 03/11] KVM: x86: dynamic kvm_apic_map

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