linux-kernel - Re: [PATCH] KVM: x86: fix APICv/x2AVIC disabled when vm reboot by itself

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Date:   Fri, 2 Dec 2022 19:10:59 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Sean Christopherson <seanjc@...gle.com>,
        Yuan ZhaoXiong <yuanzhaoxiong@...du.com>
Cc:     tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        dave.hansen@...ux.intel.com, hpa@...or.com, mlevitsk@...hat.com,
        x86@...nel.org, kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: fix APICv/x2AVIC disabled when vm reboot by
 itself

On 12/2/22 16:49, Sean Christopherson wrote:
> On Fri, Dec 02, 2022, Yuan ZhaoXiong wrote:
>> This patch fixes that VM rebooting itself will cause APICv
>> disabled when VM is started with APICv/x2AVIC enabled.
>>
>> When a VM reboot itself, The Qemu whill reset LAPIC by invoking
>> ioctl(KVM_SET_LAPIC, ...) to disable x2APIC mode and set APIC_ID
>> to its vcpuid in xAPIC mode.
>>
>> That will be handled in KVM as follows:
>>
>>       kvm_vcpu_ioctl_set_lapic
>>         kvm_apic_set_state
>> 	  kvm_lapic_set_base  =>  disable X2APIC mode
>> 	    kvm_apic_state_fixup
>> 	      kvm_lapic_xapic_id_updated
>> 	        kvm_xapic_id(apic) != apic->vcpu->vcpu_id
>> 		kvm_set_apicv_inhibit(APICV_INHIBIT_REASON_APIC_ID_MODIFIED)
>> 	   memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s))  => update APIC_ID
>>
>> kvm_apic_set_state invokes kvm_lapic_set_base to disable x2APIC mode
>> firstly, but don't change APIC_ID, APIC_ID is 32 bits in x2APIC mode
>> and 8 bist(bit 24 ~ bit 31) in xAPIC mode. So kvm_lapic_xapic_id_updated
>> will set APICV_INHIBIT_REASON_APIC_ID_MODIFIED bit inhibit and disable
>> APICv/x2AVIC.
>>
>> kvm_lapic_xapic_id_updated must be called after APIC_ID is changed.
>>
>> Fixes: 3743c2f02517 ("KVM: x86: inhibit APICv/AVIC on changes to APIC ID or APIC base")
>>
>> Signed-off-by: Yuan ZhaoXiong <yuanzhaoxiong@...du.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 d7639d1..bf5ce86 100644
>> --- a/arch/x86/kvm/lapic.c
>> +++ b/arch/x86/kvm/lapic.c
>> @@ -2722,8 +2722,6 @@ static int kvm_apic_state_fixup(struct kvm_vcpu *vcpu,
>>   			icr = __kvm_lapic_get_reg64(s->regs, APIC_ICR);
>>   			__kvm_lapic_set_reg(s->regs, APIC_ICR2, icr >> 32);
>>   		}
>> -	} else {
>> -		kvm_lapic_xapic_id_updated(vcpu->arch.apic);
>>   	}
>>   
>>   	return 0;
>> @@ -2759,6 +2757,9 @@ int kvm_apic_set_state(struct kvm_vcpu *vcpu, struct kvm_lapic_state *s)
>>   	}
>>   	memcpy(vcpu->arch.apic->regs, s->regs, sizeof(*s));
>>   
>> +	if (!apic_x2apic_mode(apic))
>> +		kvm_lapic_xapic_id_updated(apic);
>> +
> 
> Already posted[*], along with a pile of other APIC fixes.  Hopefully it will land
> in 6.2.

I'll let this patch overtake the others since it is stable@ material.

I'll also volunteer someone to write a testcase (we can add an 
apicv_enabled boolean statistic too for the sake of the test).

Paolo

> [*] https://lore.kernel.org/all/20221001005915.2041642-7-seanjc@google.com