Sender: Paolo Bonzini <paolo.bonzini@gmail.com>
Message-ID: <880a5727-69d1-72a1-b129-b053781625ad@redhat.com>
Date:   Mon, 29 Nov 2021 20:18:50 +0100
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Subject: Re: [PATCH v2 11/43] KVM: Don't block+unblock when halt-polling is
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To:     Sean Christopherson <seanjc@google.com>
Cc:     Maxim Levitsky <mlevitsk@redhat.com>,
        Marc Zyngier <maz@kernel.org>,
        Huacai Chen <chenhuacai@kernel.org>,
        Aleksandar Markovic <aleksandar.qemu.devel@gmail.com>,
        Paul Mackerras <paulus@ozlabs.org>,
        Anup Patel <anup.patel@wdc.com>,
        Paul Walmsley <paul.walmsley@sifive.com>,
        Palmer Dabbelt <palmer@dabbelt.com>,
        Albert Ou <aou@eecs.berkeley.edu>,
        Christian Borntraeger <borntraeger@de.ibm.com>,
        Janosch Frank <frankja@linux.ibm.com>,
        James Morse <james.morse@arm.com>,
        Alexandru Elisei <alexandru.elisei@arm.com>,
        Suzuki K Poulose <suzuki.poulose@arm.com>,
        Atish Patra <atish.patra@wdc.com>,
        David Hildenbrand <david@redhat.com>,
        Cornelia Huck <cohuck@redhat.com>,
        Claudio Imbrenda <imbrenda@linux.ibm.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Jim Mattson <jmattson@google.com>,
        Joerg Roedel <joro@8bytes.org>,
        linux-arm-kernel@lists.infradead.org, kvmarm@lists.cs.columbia.edu,
        linux-mips@vger.kernel.org, kvm@vger.kernel.org,
        kvm-ppc@vger.kernel.org, kvm-riscv@lists.infradead.org,
        linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org,
        David Matlack <dmatlack@google.com>,
        Oliver Upton <oupton@google.com>,
        Jing Zhang <jingzhangos@google.com>
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 <20211009021236.4122790-12-seanjc@google.com>
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 <496c2fc6-26b0-9b5d-32f4-2f9e9dd6a064@redhat.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
In-Reply-To: <YaUiEquKYi5eqWC0@google.com>
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On 11/29/21 19:55, Sean Christopherson wrote:
>> Still it does seem to be a race that happens when IS_RUNNING=true but
>> vcpu->mode == OUTSIDE_GUEST_MODE.  This patch makes the race easier to
>> trigger because it moves IS_RUNNING=false later.
> 
> Oh!  Any chance the bug only repros with preemption enabled?  That would explain
> why I don't see problems, I'm pretty sure I've only run AVIC with a PREEMPT=n.

Me too.

> svm_vcpu_{un}blocking() are called with preemption enabled, and avic_set_running()
> passes in vcpu->cpu.  If the vCPU is preempted and scheduled in on a different CPU,
> avic_vcpu_load() will overwrite the vCPU's entry with the wrong CPU info.

That would make a lot of sense.  avic_vcpu_load() can handle 
svm->avic_is_running = false, but avic_set_running still needs its body 
wrapped by preempt_disable/preempt_enable.

Fedora's kernel is CONFIG_PREEMPT_VOLUNTARY, but I know Maxim uses his 
own build so it would not surprise me if he used CONFIG_PREEMPT=y.

Paolo