linux-kernel - Re: [PATCH] KVM: x86: Properly handle APF vs disabled LAPIC situation

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Message-ID: <9a4601c7-c6a3-1b38-96de-e9d470fcaa8d@redhat.com>
Date:   Thu, 22 Apr 2021 11:40:49 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Vitaly Kuznetsov <vkuznets@...hat.com>, kvm@...r.kernel.org
Cc:     Sean Christopherson <seanjc@...gle.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Sebastien Boeuf <sebastien.boeuf@...el.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] KVM: x86: Properly handle APF vs disabled LAPIC situation

On 22/04/21 11:29, Vitaly Kuznetsov wrote:
> Async PF 'page ready' event may happen when LAPIC is (temporary) disabled.
> In particular, Sebastien reports that when Linux kernel is directly booted
> by Cloud Hypervisor, LAPIC is 'software disabled' when APF mechanism is
> initialized. On initialization KVM tries to inject 'wakeup all' event and
> puts the corresponding token to the slot. It is, however, failing to inject
> an interrupt (kvm_apic_set_irq() -> __apic_accept_irq() -> !apic_enabled())
> so the guest never gets notified and the whole APF mechanism gets stuck.
> The same issue is likely to happen if the guest temporary disables LAPIC
> and a previously unavailable page becomes available.
> 
> Do two things to resolve the issue:
> - Avoid dequeuing 'page ready' events from APF queue when LAPIC is
>    disabled.
> - Trigger an attempt to deliver pending 'page ready' events when LAPIC
>    becomes enabled (SPIV or MSR_IA32_APICBASE).
> 
> Reported-by: Sebastien Boeuf <sebastien.boeuf@...el.com>
> Signed-off-by: Vitaly Kuznetsov <vkuznets@...hat.com>
> ---
>   arch/x86/kvm/lapic.c | 6 ++++++
>   arch/x86/kvm/x86.c   | 2 +-
>   2 files changed, 7 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/lapic.c b/arch/x86/kvm/lapic.c
> index cc369b9ad8f1..49a839d0567a 100644
> --- a/arch/x86/kvm/lapic.c
> +++ b/arch/x86/kvm/lapic.c
> @@ -296,6 +296,10 @@ static inline void apic_set_spiv(struct kvm_lapic *apic, u32 val)
>   
>   		atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
>   	}
> +
> +	/* Check if there are APF page ready requests pending */
> +	if (enabled)
> +		kvm_make_request(KVM_REQ_APF_READY, apic->vcpu);
>   }
>   
>   static inline void kvm_apic_set_xapic_id(struct kvm_lapic *apic, u8 id)
> @@ -2261,6 +2265,8 @@ void kvm_lapic_set_base(struct kvm_vcpu *vcpu, u64 value)
>   		if (value & MSR_IA32_APICBASE_ENABLE) {
>   			kvm_apic_set_xapic_id(apic, vcpu->vcpu_id);
>   			static_branch_slow_dec_deferred(&apic_hw_disabled);
> +			/* Check if there are APF page ready requests pending */
> +			kvm_make_request(KVM_REQ_APF_READY, vcpu);
>   		} else {
>   			static_branch_inc(&apic_hw_disabled.key);
>   			atomic_set_release(&apic->vcpu->kvm->arch.apic_map_dirty, DIRTY);
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index a06a6f48386d..001c6a445eaf 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -11296,7 +11296,7 @@ bool kvm_arch_can_dequeue_async_page_present(struct kvm_vcpu *vcpu)
>   	if (!kvm_pv_async_pf_enabled(vcpu))
>   		return true;
>   	else
> -		return apf_pageready_slot_free(vcpu);
> +		return kvm_lapic_enabled(vcpu) && apf_pageready_slot_free(vcpu);
>   }
>   
>   void kvm_arch_start_assignment(struct kvm *kvm)
> 

Queued, with

Cc: stable@...r.kernel.org

Paolo