Re: [RFC PATCH 1/2] KVM: x86: update APIC_ID also when disabling x2APIC in kvm_lapic_set_base

[Emanuele Giuseppe Esposito]

Am 10/01/2023 um 15:07 schrieb Paolo Bonzini:
> On 1/10/23 13:16, Emanuele Giuseppe Esposito wrote:
>> I think the test in patch 2 I wrote gives a better idea on what I am
>> trying to fix: if we are transitioning from x2APIC to xAPIC (RESET I
>> would say, even though I am not sure if userspace really does it in the
>> way I do it in the test, ie through KVM_SET_MSRS), the APIC_ID is not
>> updated back in the right bits, and we can see that by querying the ID
>> with KVM_GET_LAPIC after disabling x2APIC.
>>
>> Now, if the way I reproduce this issue is correct, it is indeed a bug
>> and needs to be fixed with the fix in patch 1 or something similar.
>> I think it won't really make any difference if instead following what
>> the doc says (x2APIC -> disabled -> xAPIC) we directly do x2APIC ->
>> xAPIC.
> 
> Yes, the default value at reset is xAPIC mode, so a reset will do a
> KVM_SET_MSRS that clears X2APIC_ENABLE but leaves
> MSR_IA32_APICBASE_ENABLE set.
> 
> So, if I understand correctly...
> 
>> The test in patch 2 started being developed to test ef40757743b47 ("KVM:
>> x86: fix APICv/x2AVIC disabled when vm reboot by itself") even though I
>> honestly didn't really understand how to replicate that bug (see cover
>> letter) and instead I found this other possibility that still manages to
>> screw APIC_ID.
> 
> ... what you're saying is that there were two different bugs, but one
> fixing any one of them was enough to prevent the symptoms shown by
> commit ef40757743b47?  That is:
> 
> - the APICv inhibit was set by KVM_GET_LAPIC because it called
> kvm_lapic_xapic_id_updated(), and the call was unnecessary as fixed in
> commit ef40757743b47;
> 
> - however, there is no reason for the vCPU ID to be mismatched.  It
> happened because the code didn't handle the host-initiated x2APIC->xAPIC
> case and thus lacked a call to kvm_apic_set_xapic_id().
> 
> If so, I think the idea of the patch is fine.

Yes :)
> 
> Just one thing: your patch also changes the APIC_ID on the
> x2APIC->disabled transition, not just the "forbidden" (i.e. host-
> initiated only) x2APIC->xAPIC transition.  I think  this is okay too: the
> manual says:
> 
>    10.4.3 Enabling or Disabling the Local APIC
> 
>    When IA32_APIC_BASE[11] is set to 0, prior initialization to the APIC
>    may be lost and the APIC may return to the state described in Section
>    10.4.7.1, “Local APIC State After Power-Up or Reset.”
> 
>    10.4.7.1 Local APIC State After Power-Up or Reset
> 
>    ... The local APIC ID register is set to a unique APIC ID. ...
> 
> (which must be an xAPIC ID) and this is what your patch does.
> 
> In fact perhaps you can change the code further to invoke
> kvm_lapic_reset() after static_branch_inc(&apic_hw_disabled.key)? 

Ok, it makes sense. In that case we are disabling lapic (X2APIC_ENABLE
should be 0 too, otherwise it would be invalid.

xAPIC global enable(IA32_APIC_BASE[11]) | x2APIC
enable(IA32_APIC_BASE[10]) | Description
0 0 local APIC is disabled
0 1 Invalid
1 0 local APIC is enabled in xAPIC mode
1 1 local APIC is enabled in x2APIC mode

Thank you,
Emanuele

 It's
> just a bit messy that you have a call back to kvm_lapic_set_base() in
> there, so perhaps something like this can help:
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index 4efdb4a4d72c..24e5df23a4d9 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -2433,9 +2436,7 @@ void kvm_apic_update_apicv(struct kvm_vcpu *vcpu)
>  
>  void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
>  {
> -    struct kvm_lapic *apic = vcpu->arch.apic;
>      u64 msr_val;
> -    int i;
>  
>      if (!init_event) {
>          msr_val = APIC_DEFAULT_PHYS_BASE | MSR_IA32_APICBASE_ENABLE;
> @@ -2444,8 +2445,14 @@ void kvm_lapic_reset(struct kvm_vcpu *vcpu, bool
> init_event)
>          kvm_lapic_set_base(vcpu, msr_val);
>      }
>  
> -    if (!apic)
> -        return;
> +    if (vcpu->arch.apic)
> +        __kvm_lapic_reset(vcpu, init_event);
> +}
> +
> +static void __kvm_lapic_reset(struct kvm_vcpu *vcpu, bool init_event)
> +{
> +    struct kvm_lapic *apic = vcpu->arch.apic;
> +    int i;
>  
>      /* Stop the timer in case it's a reset to an active apic */
>      hrtimer_cancel(&apic->lapic_timer.timer);
> 
> 
> (just a sketch to show the idea, of course __kvm_lapic_reset would have to
> go first).
> 
> Paolo
>