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Subject: Re: [PATCH 02/10] KVM: x86: APICv: fix race in
 kvm_request_apicv_update on SVM
To:     Maxim Levitsky <mlevitsk@redhat.com>, kvm@vger.kernel.org
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Sean Christopherson <seanjc@google.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Joerg Roedel <joro@8bytes.org>, Borislav Petkov <bp@alien8.de>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@redhat.com>,
        "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" 
        <linux-kernel@vger.kernel.org>,
        "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" <x86@kernel.org>,
        Jim Mattson <jmattson@google.com>
References: <20210623113002.111448-1-mlevitsk@redhat.com>
 <20210623113002.111448-3-mlevitsk@redhat.com>
From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <6c4a69ce-595e-d5a1-7b4e-e6ce1afe1252@redhat.com>
Date:   Wed, 23 Jun 2021 23:50:35 +0200
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On 23/06/21 13:29, Maxim Levitsky wrote:
> +	kvm_block_guest_entries(kvm);
> +
>   	trace_kvm_apicv_update_request(activate, bit);
>   	if (kvm_x86_ops.pre_update_apicv_exec_ctrl)
>   		static_call(kvm_x86_pre_update_apicv_exec_ctrl)(kvm, activate);
> @@ -9243,6 +9245,8 @@ void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
>   	except = kvm_get_running_vcpu();
>   	kvm_make_all_cpus_request_except(kvm, KVM_REQ_APICV_UPDATE,
>   					 except);
> +
> +	kvm_allow_guest_entries(kvm);

Doesn't this cause a busy loop during synchronize_rcu?  It should be 
possible to request the vmexit of other CPUs from 
avic_update_access_page, and do a lock/unlock of kvm->slots_lock to wait 
for the memslot to be updated.

(As an aside, I'd like to get rid of KVM_REQ_MCLOCK_IN_PROGRESS in 5.15...).

Paolo