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([2001:b07:6468:f312:c8dd:75d4:99ab:290a]) by smtp.gmail.com with ESMTPSA id w3sm2879892eds.78.2021.05.24.09.21.17 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Mon, 24 May 2021 09:21:18 -0700 (PDT) Subject: Re: [PATCH v2 5/5] KVM: x86: hyper-v: Deactivate APICv only when AutoEOI feature is in use To: Vitaly Kuznetsov , kvm@vger.kernel.org Cc: Sean Christopherson , Wanpeng Li , Jim Mattson , Maxim Levitsky , Kechen Lu , linux-kernel@vger.kernel.org References: <20210518144339.1987982-1-vkuznets@redhat.com> <20210518144339.1987982-6-vkuznets@redhat.com> From: Paolo Bonzini Message-ID: <82e2a6a0-337a-4b92-2271-493344a38960@redhat.com> Date: Mon, 24 May 2021 18:21:17 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:78.0) Gecko/20100101 Thunderbird/78.8.1 MIME-Version: 1.0 In-Reply-To: <20210518144339.1987982-6-vkuznets@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: linux-kernel@vger.kernel.org On 18/05/21 16:43, Vitaly Kuznetsov wrote: > APICV_INHIBIT_REASON_HYPERV is currently unconditionally forced upon > SynIC activation as SynIC's AutoEOI is incompatible with APICv/AVIC. It is, > however, possible to track whether the feature was actually used by the > guest and only inhibit APICv/AVIC when needed. > > TLFS suggests a dedicated 'HV_DEPRECATING_AEOI_RECOMMENDED' flag to let > Windows know that AutoEOI feature should be avoided. While it's up to > KVM userspace to set the flag, KVM can help a bit by exposing global > APICv/AVIC enablement: in case APICv/AVIC usage is impossible, AutoEOI > is still preferred. > > Signed-off-by: Vitaly Kuznetsov Should it also disable APICv unconditionally if HV_DEPRECATING_AEOI_RECOMMENDED is not in the guest CPUID? That should avoid ping-pong between enabled and disabled APICv even in pathological cases that we cannot think about. Paolo > --- > arch/x86/include/asm/kvm_host.h | 3 +++ > arch/x86/kvm/hyperv.c | 27 +++++++++++++++++++++------ > 2 files changed, 24 insertions(+), 6 deletions(-) > > diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h > index bf5807d35339..5e03ab4c0e4f 100644 > --- a/arch/x86/include/asm/kvm_host.h > +++ b/arch/x86/include/asm/kvm_host.h > @@ -936,6 +936,9 @@ struct kvm_hv { > /* How many vCPUs have VP index != vCPU index */ > atomic_t num_mismatched_vp_indexes; > > + /* How many SynICs use 'AutoEOI' feature */ > + atomic_t synic_auto_eoi_used; > + > struct hv_partition_assist_pg *hv_pa_pg; > struct kvm_hv_syndbg hv_syndbg; > }; > diff --git a/arch/x86/kvm/hyperv.c b/arch/x86/kvm/hyperv.c > index f98370a39936..89e7d5b99279 100644 > --- a/arch/x86/kvm/hyperv.c > +++ b/arch/x86/kvm/hyperv.c > @@ -87,6 +87,10 @@ static bool synic_has_vector_auto_eoi(struct kvm_vcpu_hv_synic *synic, > static void synic_update_vector(struct kvm_vcpu_hv_synic *synic, > int vector) > { > + struct kvm_vcpu *vcpu = hv_synic_to_vcpu(synic); > + struct kvm_hv *hv = to_kvm_hv(vcpu->kvm); > + int auto_eoi_old, auto_eoi_new; > + > if (vector < HV_SYNIC_FIRST_VALID_VECTOR) > return; > > @@ -95,10 +99,25 @@ static void synic_update_vector(struct kvm_vcpu_hv_synic *synic, > else > __clear_bit(vector, synic->vec_bitmap); > > + auto_eoi_old = bitmap_weight(synic->auto_eoi_bitmap, 256); > + > if (synic_has_vector_auto_eoi(synic, vector)) > __set_bit(vector, synic->auto_eoi_bitmap); > else > __clear_bit(vector, synic->auto_eoi_bitmap); > + > + auto_eoi_new = bitmap_weight(synic->auto_eoi_bitmap, 256); > + > + /* Hyper-V SynIC auto EOI SINTs are not compatible with APICV */ > + if (!auto_eoi_old && auto_eoi_new) { > + if (atomic_inc_return(&hv->synic_auto_eoi_used) == 1) > + kvm_request_apicv_update(vcpu->kvm, false, > + APICV_INHIBIT_REASON_HYPERV); > + } else if (!auto_eoi_new && auto_eoi_old) { > + if (atomic_dec_return(&hv->synic_auto_eoi_used) == 0) > + kvm_request_apicv_update(vcpu->kvm, true, > + APICV_INHIBIT_REASON_HYPERV); > + } > } > > static int synic_set_sint(struct kvm_vcpu_hv_synic *synic, int sint, > @@ -931,12 +950,6 @@ int kvm_hv_activate_synic(struct kvm_vcpu *vcpu, bool dont_zero_synic_pages) > > synic = to_hv_synic(vcpu); > > - /* > - * Hyper-V SynIC auto EOI SINT's are > - * not compatible with APICV, so request > - * to deactivate APICV permanently. > - */ > - kvm_request_apicv_update(vcpu->kvm, false, APICV_INHIBIT_REASON_HYPERV); > synic->active = true; > synic->dont_zero_synic_pages = dont_zero_synic_pages; > synic->control = HV_SYNIC_CONTROL_ENABLE; > @@ -2198,6 +2211,8 @@ int kvm_get_hv_cpuid(struct kvm_vcpu *vcpu, struct kvm_cpuid2 *cpuid, > ent->eax |= HV_X64_ENLIGHTENED_VMCS_RECOMMENDED; > if (!cpu_smt_possible()) > ent->eax |= HV_X64_NO_NONARCH_CORESHARING; > + if (enable_apicv) > + ent->eax |= HV_DEPRECATING_AEOI_RECOMMENDED; > /* > * Default number of spinlock retry attempts, matches > * HyperV 2016. >