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    SubjectRe: [RFC PATCH v3 02/19] KVM: x86: inhibit APICv/AVIC when the guest and/or host changes apic id/base from the defaults.
    On Wed, May 18, 2022 at 12:50:27PM +0300, Maxim Levitsky wrote:
    >> > struct kvm_arch {
    >> > @@ -1258,6 +1260,7 @@ struct kvm_arch {
    >> > hpa_t hv_root_tdp;
    >> > spinlock_t hv_root_tdp_lock;
    >> > #endif
    >> > + bool apic_id_changed;
    >>
    >> What's the value of this boolean? No one reads it.
    >
    >I use it in later patches to kill the guest during nested VM entry
    >if it attempts to use nested AVIC after any vCPU changed APIC ID.
    >
    >I mentioned this boolean in the commit description.
    >
    >This boolean avoids the need to go over all vCPUs and checking
    >if they still have the initial apic id.

    Do you want to kill the guest if APIC base got changed? If yes,
    you can check if APICV_INHIBIT_REASON_RO_SETTINGS is set and save
    the boolean.

    >
    >In the future maybe we can introduce a more generic 'taint'
    >bitmap with various flags like that, indicating that the guest
    >did something unexpected.
    >
    >BTW, the other option in regard to the nested AVIC is just to ignore this issue completely.
    >The code itself always uses vcpu_id's, thus regardless of when/how often the guest changes
    >its apic ids, my code would just use the initial APIC ID values consistently.
    >
    >In this case I won't need this boolean.
    >
    >>
    >> > };
    >> >
    >> > struct kvm_vm_stat {
    >> > diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
    >> > index 66b0eb0bda94e..8996675b3ef4c 100644
    >> > --- a/arch/x86/kvm/lapic.c
    >> > +++ b/arch/x86/kvm/lapic.c
    >> > @@ -2038,6 +2038,19 @@ static void apic_manage_nmi_watchdog(struct kvm_lapic *apic, u32 lvt0_val)
    >> > }
    >> > }
    >> >
    >> > +static void kvm_lapic_check_initial_apic_id(struct kvm_lapic *apic)
    >> > +{
    >> > + if (kvm_apic_has_initial_apic_id(apic))
    >> > + return;
    >> > +
    >> > + pr_warn_once("APIC ID change is unsupported by KVM");
    >>
    >> It is misleading because changing xAPIC ID is supported by KVM; it just
    >> isn't compatible with APICv. Probably this pr_warn_once() should be
    >> removed.
    >
    >Honestly since nobody uses this feature, I am not sure if to call this supported,
    >I am sure that KVM has more bugs in regard of using non standard APIC ID.
    >This warning might hopefuly make someone complain about it if this
    >feature is actually used somewhere.

    Now I got you. It is fine to me.

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