Re: [RFC PATCH v3 02/19] KVM: x86: inhibit APICv/AVIC when the guest and/or host changes apic id/base from the defaults.

Chao Gao
2022-05-18T11:51:24Z

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.