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Date:   Thu, 5 May 2022 08:38:08 +0700
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Subject: Re: [PATCH v3 08/14] KVM: SVM: Update AVIC settings when changing
 APIC mode
Content-Language: en-US
To:     Maxim Levitsky <mlevitsk@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     pbonzini@redhat.com, seanjc@google.com, joro@8bytes.org,
        jon.grimm@amd.com, wei.huang2@amd.com, terry.bowman@amd.com
References: <20220504073128.12031-1-suravee.suthikulpanit@amd.com>
 <20220504073128.12031-9-suravee.suthikulpanit@amd.com>
 <ff67344c0efe06d1422aa84e56738a0812c69bfc.camel@redhat.com>
From:   Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
In-Reply-To: <ff67344c0efe06d1422aa84e56738a0812c69bfc.camel@redhat.com>
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Maxim,

On 5/4/22 7:19 PM, Maxim Levitsky wrote:
> On Wed, 2022-05-04 at 02:31 -0500, Suravee Suthikulpanit wrote:
>> Update and refresh AVIC settings when guest APIC mode is updated
>> (e.g. changing between disabled, xAPIC, or x2APIC).
>>
>> Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@amd.com>
>> ---
>>   arch/x86/kvm/svm/avic.c | 16 ++++++++++++++++
>>   arch/x86/kvm/svm/svm.c  |  1 +
>>   2 files changed, 17 insertions(+)
>>
>> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
>> index 3ebeea19b487..d185dd8ddf17 100644
>> --- a/arch/x86/kvm/svm/avic.c
>> +++ b/arch/x86/kvm/svm/avic.c
>> @@ -691,6 +691,22 @@ void avic_apicv_post_state_restore(struct kvm_vcpu *vcpu)
>>   	avic_handle_ldr_update(vcpu);
>>   }
>>   
>> +void avic_set_virtual_apic_mode(struct kvm_vcpu *vcpu)
>> +{
>> +	struct vcpu_svm *svm = to_svm(vcpu);
>> +
>> +	if (!lapic_in_kernel(vcpu) || (avic_mode == AVIC_MODE_NONE))
>> +		return;
>> +
>> +	if (kvm_get_apic_mode(vcpu) == LAPIC_MODE_INVALID) {
>> +		WARN_ONCE(true, "Invalid local APIC state (vcpu_id=%d)", vcpu->vcpu_id);
>> +		return;
>> +	}
>> +
>> +	kvm_vcpu_update_apicv(&svm->vcpu);
> Why to have this call? I think that all that is needed is only to call the
> avic_refresh_apicv_exec_ctrl.

When APIC mode is updated on each vCPU, we need to check and update
vcpu->arch.apicv_active accordingly, which happens in the kvm_vcpu_update_apicv()

One test case that would fail w/o the kvm_vcpu_update_apicv() is when
we boot a Linux guest w/ guest kernel option _nox2apic_, which Linux forces APIC
mode of vCPUs with APIC ID 255 and higher to disable. W/o this line of code, the VM
would not boot w/ more than 255 vCPUs.

Regards,
Suravee