linux-kernel - Re: [PATCH v3 07/14] KVM: x86: reset APIC ID when enabling LAPIC

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Date:	Tue, 2 Aug 2016 15:52:03 +0200
From:	Radim Krčmář <rkrcmar@...hat.com>
To:	Wanpeng Li <kernellwp@...il.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
	"Lan, Tianyu" <tianyu.lan@...el.com>,
	Igor Mammedov <imammedo@...hat.com>,
	Jan Kiszka <jan.kiszka@....de>, Peter Xu <peterx@...hat.com>,
	Yang Zhang <yang.zhang.wz@...il.com>
Subject: Re: [PATCH v3 07/14] KVM: x86: reset APIC ID when enabling LAPIC

2016-08-02 21:26+0800, Wanpeng Li:
> 2016-07-13 4:09 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
>> APIC ID should be set to the initial APIC ID when enabling LAPIC.
>> This only matters if the guest changes APIC ID.  No sane OS does that.
>>
>> Signed-off-by: Radim Krčmář <rkrcmar@...hat.com>
>> ---
>>  arch/x86/kvm/lapic.c | 5 +++--
>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
>> index 41089dbeeafc..0fce77fdbe91 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -1720,9 +1720,10 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
>>
>>         /* update jump label if enable bit changes */
>>         if ((old_value ^ value) & MSR_IA32_APICBASE_ENABLE) {
>> -               if (value & MSR_IA32_APICBASE_ENABLE)
>> +               if (value & MSR_IA32_APICBASE_ENABLE) {
>> +                       kvm_apic_set_xapic_id(apic, vcpu->vcpu_id);
>>                         static_key_slow_dec_deferred(&apic_hw_disabled);
>> -               else
>> +               } else
>>                         static_key_slow_inc(&apic_hw_disabled.key);
>>                 recalculate_apic_map(vcpu->kvm);
>                    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> If we can drop this recalculate_apic_map() since it has already been
> called by kvm_apic_set_xapic_id().

Good point.  We would need to call it in the else branch, though.