KVM: x86: don't disable APICv memslot when inhibited

Thanks to the former patches, it is now possible to keep the APICv
memslot always enabled, and it will be invisible to the guest
when it is inhibited

This code is based on a suggestion from Sean Christopherson:
https://lkml.org/lkml/2021/7/19/2970

Suggested-by: Sean Christopherson <[e-mail]>
Signed-off-by: Maxim Levitsky <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/include/asm/kvm-x86-ops.h

···
 KVM_X86_OP(enable_irq_window)
 KVM_X86_OP(update_cr8_intercept)
 KVM_X86_OP(check_apicv_inhibit_reasons)
-KVM_X86_OP_NULL(pre_update_apicv_exec_ctrl)
 KVM_X86_OP(refresh_apicv_exec_ctrl)
 KVM_X86_OP(hwapic_irr_update)
 KVM_X86_OP(hwapic_isr_update)

arch/x86/include/asm/kvm_host.h

···
 	void (*enable_irq_window)(struct kvm_vcpu *vcpu);
 	void (*update_cr8_intercept)(struct kvm_vcpu *vcpu, int tpr, int irr);
 	bool (*check_apicv_inhibit_reasons)(ulong bit);
-	void (*pre_update_apicv_exec_ctrl)(struct kvm *kvm, bool activate);
 	void (*refresh_apicv_exec_ctrl)(struct kvm_vcpu *vcpu);
 	void (*hwapic_irr_update)(struct kvm_vcpu *vcpu, int max_irr);
 	void (*hwapic_isr_update)(struct kvm_vcpu *vcpu, int isr);

arch/x86/kvm/svm/avic.c

···
  * field of the VMCB. Therefore, we set up the
  * APIC_ACCESS_PAGE_PRIVATE_MEMSLOT (4KB) here.
  */
-static int avic_update_access_page(struct kvm *kvm, bool activate)
+static int avic_alloc_access_page(struct kvm *kvm)
 {
 	void __user *ret;
 	int r = 0;
 
 	mutex_lock(&kvm->slots_lock);
-	/*
-	 * During kvm_destroy_vm(), kvm_pit_set_reinject() could trigger
-	 * APICv mode change, which update APIC_ACCESS_PAGE_PRIVATE_MEMSLOT
-	 * memory region. So, we need to ensure that kvm->mm == current->mm.
-	 */
-	if ((kvm->arch.apic_access_memslot_enabled == activate) ||
-	    (kvm->mm != current->mm))
+
+	if (kvm->arch.apic_access_memslot_enabled)
 		goto out;
 
 	ret = __x86_set_memory_region(kvm,
 				      APIC_ACCESS_PAGE_PRIVATE_MEMSLOT,
 				      APIC_DEFAULT_PHYS_BASE,
-				      activate ? PAGE_SIZE : 0);
+				      PAGE_SIZE);
 	if (IS_ERR(ret)) {
 		r = PTR_ERR(ret);
 		goto out;
 	}
 
-	kvm->arch.apic_access_memslot_enabled = activate;
+	kvm->arch.apic_access_memslot_enabled = true;
 out:
 	mutex_unlock(&kvm->slots_lock);
 	return r;
···
 	if (kvm_apicv_activated(vcpu->kvm)) {
 		int ret;
 
-		ret = avic_update_access_page(vcpu->kvm, true);
+		ret = avic_alloc_access_page(vcpu->kvm);
 		if (ret)
 			return ret;
 	}
···
 	return supported & BIT(bit);
 }
 
-void svm_pre_update_apicv_exec_ctrl(struct kvm *kvm, bool activate)
-{
-	avic_update_access_page(kvm, activate);
-}
 
 static inline int
 avic_update_iommu_vcpu_affinity(struct kvm_vcpu *vcpu, int cpu, bool r)

arch/x86/kvm/svm/svm.c

···
 	.set_virtual_apic_mode = svm_set_virtual_apic_mode,
 	.refresh_apicv_exec_ctrl = svm_refresh_apicv_exec_ctrl,
 	.check_apicv_inhibit_reasons = svm_check_apicv_inhibit_reasons,
-	.pre_update_apicv_exec_ctrl = svm_pre_update_apicv_exec_ctrl,
 	.load_eoi_exitmap = svm_load_eoi_exitmap,
 	.hwapic_irr_update = svm_hwapic_irr_update,
 	.hwapic_isr_update = svm_hwapic_isr_update,

arch/x86/kvm/svm/svm.h

···
 void svm_set_virtual_apic_mode(struct kvm_vcpu *vcpu);
 void svm_refresh_apicv_exec_ctrl(struct kvm_vcpu *vcpu);
 bool svm_check_apicv_inhibit_reasons(ulong bit);
-void svm_pre_update_apicv_exec_ctrl(struct kvm *kvm, bool activate);
 void svm_load_eoi_exitmap(struct kvm_vcpu *vcpu, u64 *eoi_exit_bitmap);
 void svm_hwapic_irr_update(struct kvm_vcpu *vcpu, int max_irr);
 void svm_hwapic_isr_update(struct kvm_vcpu *vcpu, int max_isr);

arch/x86/kvm/x86.c

···
 }
 EXPORT_SYMBOL_GPL(kvm_vcpu_update_apicv);
 
-/*
- * NOTE: Do not hold any lock prior to calling this.
- *
- * In particular, kvm_request_apicv_update() expects kvm->srcu not to be
- * locked, because it calls __x86_set_memory_region() which does
- * synchronize_srcu(&kvm->srcu).
- */
 void kvm_request_apicv_update(struct kvm *kvm, bool activate, ulong bit)
 {
 	unsigned long old, new, expected;
···
 		old = cmpxchg(&kvm->arch.apicv_inhibit_reasons, expected, new);
 	} while (old != expected);
 
-	if (!!old == !!new)
-		return;
+	if (!!old != !!new) {
+		trace_kvm_apicv_update_request(activate, bit);
+		kvm_make_all_cpus_request(kvm, KVM_REQ_APICV_UPDATE);
+		if (new) {
+			unsigned long gfn = gpa_to_gfn(APIC_DEFAULT_PHYS_BASE);
 
-	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);
+			kvm_zap_gfn_range(kvm, gfn, gfn+1);
+		}
+	}
 
-	kvm_make_all_cpus_request(kvm, KVM_REQ_APICV_UPDATE);
 }
 EXPORT_SYMBOL_GPL(kvm_request_apicv_update);