linux-kernel - Re: [PATCH v2 08/12] KVM: SVM: Update AVIC settings when changing APIC mode

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Message-ID: <9307c734-3473-0bdc-57be-c39e96bca4d8@amd.com>
Date:   Mon, 2 May 2022 21:07:44 +0700
From:   Suravee Suthikulpanit <suravee.suthikulpanit@....com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     pbonzini@...hat.com, seanjc@...gle.com, joro@...tes.org,
        jon.grimm@....com, wei.huang2@....com, terry.bowman@....com
Subject: Re: [PATCH v2 08/12] KVM: SVM: Update AVIC settings when changing
 APIC mode

Maxim, Sean

On 4/18/22 7:55 PM, Maxim Levitsky wrote:
> On Tue, 2022-04-12 at 06:58 -0500, Suravee Suthikulpanit wrote:
>> When APIC mode is updated (e.g. disabled, xAPIC, or x2APIC),
>> KVM needs to call kvm_vcpu_update_apicv() to update AVIC settings
>> accordingly.
>>
>> Signed-off-by: Suravee Suthikulpanit<suravee.suthikulpanit@....com>
>> ---
>>   arch/x86/kvm/svm/avic.c | 15 +++++++++++++++
>>   arch/x86/kvm/svm/svm.c  |  1 +
>>   2 files changed, 16 insertions(+)
>>
>> diff --git a/arch/x86/kvm/svm/avic.c b/arch/x86/kvm/svm/avic.c
>> index 22ee1098e2a5..01392b8364f4 100644
>> --- a/arch/x86/kvm/svm/avic.c
>> +++ b/arch/x86/kvm/svm/avic.c
>> @@ -616,6 +616,21 @@ 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);
> I think it makes sense to call avic_refresh_apicv_exec_ctrl directly here.
>   
> I am not sure that kvm_vcpu_update_apicv will even call it
> because it has an optimization of doing nothing when inhibition status
> didn't change.
>   
>   
> Another semi-related note:
>   
> the current way the x2avic msrs are configured creates slight performance
> problem for nesting:
>   
> The problem is that when entering a nested guest, AVIC on the current vCPU
> is inhibited, but this is done only so that this vCPU*peers*  don't
> try to use AVIC to send IPIs to it, so there is no need to update vmcb01
> msr interception bitmap, and vmcb02 should have all these msrs intercepted always.
> Same with returning to host.
> 
> It also should be checked that during nested entry, at least vmcb01 msr bitmap
> is updated - TL;DR - please check that x2avic works when there is a nested guest running.

In the kvm/queue branch, I found a regression on nested SVM guest, where L2 guest cannot
launch. The bad commit is:

commit a4cfff3f0f8c07f1f7873a82bdeb3995807dac8c (bisect)
Merge: 42dcbe7d8bac 8d5678a76689
Author: Paolo Bonzini <pbonzini@...hat.com>
Date:   Fri Apr 8 12:43:40 2022 -0400

     Merge branch 'kvm-older-features' into HEAD

     Merge branch for features that did not make it into 5.18:

     * New ioctls to get/set TSC frequency for a whole VM

     * Allow userspace to opt out of hypercall patching

     Nested virtualization improvements for AMD:

     * Support for "nested nested" optimizations (nested vVMLOAD/VMSAVE,
       nested vGIF)

     * Allow AVIC to co-exist with a nested guest running

     * Fixes for LBR virtualizations when a nested guest is running,
       and nested LBR virtualization support

     * PAUSE filtering for nested hypervisors

     Guest support:

     * Decoupling of vcpu_is_preempted from PV spinlocks

     Signed-off-by: Paolo Bonzini <pbonzini@...hat.com>

I am still working on the bisect into the merge commits.

Regards,
Suravee