linux-kernel - Re: [PATCH v2 5/8] KVM: x86: APICv: fix race in kvm_request_apicv

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Date:   Tue, 27 Jul 2021 00:34:54 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>, kvm@...r.kernel.org
Cc:     "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@...r.kernel.org>, Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>, Borislav Petkov <bp@...en8.de>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...hat.com>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@...nel.org>,
        Sean Christopherson <seanjc@...gle.com>
Subject: Re: [PATCH v2 5/8] KVM: x86: APICv: fix race in
 kvm_request_apicv_update on SVM

On 13/07/21 16:20, Maxim Levitsky wrote:
> +	mutex_lock(&vcpu->kvm->apicv_update_lock);
> +
>  	vcpu->arch.apicv_active = kvm_apicv_activated(vcpu->kvm);
>  	kvm_apic_update_apicv(vcpu);
>  	static_call(kvm_x86_refresh_apicv_exec_ctrl)(vcpu);
> @@ -9246,6 +9248,8 @@ void kvm_vcpu_update_apicv(struct kvm_vcpu *vcpu)
>  	 */
>  	if (!vcpu->arch.apicv_active)
>  		kvm_make_request(KVM_REQ_EVENT, vcpu);
> +
> +	mutex_unlock(&vcpu->kvm->apicv_update_lock);

Does this whole piece of code need the lock/unlock?  Does it work and/or 
make sense to do the unlock immediately after mutex_lock()?  This makes 
it clearer that the mutex is being to synchronize against the requestor.

> diff --git a/virt/kvm/kvm_main.c b/virt/kvm/kvm_main.c
> index ed4d1581d502..ba5d5d9ebc64 100644
> --- a/virt/kvm/kvm_main.c
> +++ b/virt/kvm/kvm_main.c
> @@ -943,6 +943,7 @@ static struct kvm *kvm_create_vm(unsigned long type)
>   	mutex_init(&kvm->irq_lock);
>   	mutex_init(&kvm->slots_lock);
>   	mutex_init(&kvm->slots_arch_lock);
> +	mutex_init(&kvm->apicv_update_lock);
>   	INIT_LIST_HEAD(&kvm->devices);
>   
>   	BUILD_BUG_ON(KVM_MEM_SLOTS_NUM > SHRT_MAX);
> 

Please add comments above fields that are protected by this lock 
(anything but apicv_inhibit_reasons?), and especially move it to kvm->arch.

Paolo