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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id r23sm720630edy.13.2021.06.23.14.50.35 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 23 Jun 2021 14:50:36 -0700 (PDT) Subject: Re: [PATCH 02/10] KVM: x86: APICv: fix race in kvm_request_apicv_update on SVM To: Maxim Levitsky , kvm@vger.kernel.org Cc: Thomas Gleixner , Sean Christopherson , Wanpeng Li , Vitaly Kuznetsov , Joerg Roedel , Borislav Petkov , "H. Peter Anvin" , Ingo Molnar , "open list:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , "maintainer:X86 ARCHITECTURE (32-BIT AND 64-BIT)" , Jim Mattson References: <20210623113002.111448-1-mlevitsk@redhat.com> <20210623113002.111448-3-mlevitsk@redhat.com> From: Paolo Bonzini Message-ID: <6c4a69ce-595e-d5a1-7b4e-e6ce1afe1252@redhat.com> Date: Wed, 23 Jun 2021 23:50:35 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.10.1 MIME-Version: 1.0 In-Reply-To: <20210623113002.111448-3-mlevitsk@redhat.com> Content-Type: text/plain; charset=utf-8; format=flowed Content-Language: en-US Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: kvm@vger.kernel.org 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