linux-kernel - Re: [PATCH v3] KVM: LAPIC: Recalculate apic map in batch

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Message-ID: <cc8b429e-74dd-70f1-8f1c-8893a5485e76@redhat.com>
Date:   Mon, 22 Jun 2020 16:37:06 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Igor Mammedov <imammedo@...hat.com>
Cc:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org,
        Sean Christopherson <sean.j.christopherson@...el.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Jim Mattson <jmattson@...gle.com>,
        Joerg Roedel <joro@...tes.org>
Subject: Re: [PATCH v3] KVM: LAPIC: Recalculate apic map in batch

On 22/06/20 00:26, Igor Mammedov wrote:
> 
> following sequence looks like a race that can cause lost map update events:
> 
>          cpu1                            cpu2
>                              
>                                 apic_map_dirty = true     
>   ------------------------------------------------------------   
>                                 kvm_recalculate_apic_map:
>                                      pass check
>                                          mutex_lock(&kvm->arch.apic_map_lock);
>                                          if (!kvm->arch.apic_map_dirty)
>                                      and in process of updating map
>   -------------------------------------------------------------
>     other calls to
>        apic_map_dirty = true         might be too late for affected cpu
>   -------------------------------------------------------------
>                                      apic_map_dirty = false
>   -------------------------------------------------------------
>     kvm_recalculate_apic_map:
>     bail out on
>       if (!kvm->arch.apic_map_dirty)

I will post a fix for that.  Thanks for the analysis!

Paolo