linux-kernel - Re: [RFC PATCH] KVM: x86: Disallow KVM_SET

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Date:   Wed, 18 Dec 2019 11:38:43 -0800
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Weijiang Yang <weijiang.yang@...el.com>
Subject: Re: [RFC PATCH] KVM: x86: Disallow KVM_SET_CPUID{2} if the vCPU is in
 guest mode

On Wed, Dec 18, 2019 at 9:42 AM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> Reject KVM_SET_CPUID{2} with -EBUSY if the vCPU is in guest mode (L2) to
> avoid complications and potentially undesirable KVM behavior.  Allowing
> userspace to change a guest's capabilities while L2 is active would at
> best result in unexpected behavior in the guest (L1 or L2), and at worst
> induce bad KVM behavior by breaking fundamental assumptions regarding
> transitions between L0, L1 and L2.

This seems a bit contrived. As long as we're breaking the ABI, can we
disallow changes to CPUID once the vCPU has been powered on?