KVM: x86: Move Hyper-V partition assist page out of Hyper-V emulation context

Hyper-V partition assist page is used when KVM runs on top of Hyper-V and
is not used for Windows/Hyper-V guests on KVM, this means that 'hv_pa_pg'
placement in 'struct kvm_hv' is unfortunate. As a preparation to making
Hyper-V emulation optional, move 'hv_pa_pg' to 'struct kvm_arch' and put it
under CONFIG_HYPERV.

While on it, introduce hv_get_partition_assist_page() helper to allocate
partition assist page. Move the comment explaining why we use a single page
for all vCPUs from VMX and expand it a bit.

No functional change intended.

Reviewed-by: Maxim Levitsky <[e-mail]>
Tested-by: Jeremi Piotrowski <[e-mail]>
Signed-off-by: Vitaly Kuznetsov <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

arch/x86/include/asm/kvm_host.h

···
 	 */
 	unsigned int synic_auto_eoi_used;
 
-	struct hv_partition_assist_pg *hv_pa_pg;
 	struct kvm_hv_syndbg hv_syndbg;
 };
 
···
 #if IS_ENABLED(CONFIG_HYPERV)
 	hpa_t	hv_root_tdp;
 	spinlock_t hv_root_tdp_lock;
+	struct hv_partition_assist_pg *hv_pa_pg;
 #endif
 	/*
 	 * VM-scope maximum vCPU ID. Used to determine the size of structures

arch/x86/kvm/kvm_onhyperv.h

···
 int hv_flush_remote_tlbs_range(struct kvm *kvm, gfn_t gfn, gfn_t nr_pages);
 int hv_flush_remote_tlbs(struct kvm *kvm);
 void hv_track_root_tdp(struct kvm_vcpu *vcpu, hpa_t root_tdp);
+static inline hpa_t hv_get_partition_assist_page(struct kvm_vcpu *vcpu)
+{
+	/*
+	 * Partition assist page is something which Hyper-V running in L0
+	 * requires from KVM running in L1 before direct TLB flush for L2
+	 * guests can be enabled. KVM doesn't currently use the page but to
+	 * comply with TLFS it still needs to be allocated. For now, this
+	 * is a single page shared among all vCPUs.
+	 */
+	struct hv_partition_assist_pg **p_hv_pa_pg =
+		&vcpu->kvm->arch.hv_pa_pg;
+
+	if (!*p_hv_pa_pg)
+		*p_hv_pa_pg = kzalloc(PAGE_SIZE, GFP_KERNEL_ACCOUNT);
+
+	if (!*p_hv_pa_pg)
+		return INVALID_PAGE;
+
+	return __pa(*p_hv_pa_pg);
+}
 #else /* !CONFIG_HYPERV */
 static inline int hv_flush_remote_tlbs(struct kvm *kvm)
 {

arch/x86/kvm/svm/svm_onhyperv.c

···
 int svm_hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu)
 {
 	struct hv_vmcb_enlightenments *hve;
-	struct hv_partition_assist_pg **p_hv_pa_pg =
-			&to_kvm_hv(vcpu->kvm)->hv_pa_pg;
+	hpa_t partition_assist_page = hv_get_partition_assist_page(vcpu);
 
-	if (!*p_hv_pa_pg)
-		*p_hv_pa_pg = kzalloc(PAGE_SIZE, GFP_KERNEL);
-
-	if (!*p_hv_pa_pg)
+	if (partition_assist_page == INVALID_PAGE)
 		return -ENOMEM;
 
 	hve = &to_svm(vcpu)->vmcb->control.hv_enlightenments;
 
-	hve->partition_assist_page = __pa(*p_hv_pa_pg);
+	hve->partition_assist_page = partition_assist_page;
 	hve->hv_vm_id = (unsigned long)vcpu->kvm;
 	if (!hve->hv_enlightenments_control.nested_flush_hypercall) {
 		hve->hv_enlightenments_control.nested_flush_hypercall = 1;

arch/x86/kvm/vmx/vmx.c

···
 static int hv_enable_l2_tlb_flush(struct kvm_vcpu *vcpu)
 {
 	struct hv_enlightened_vmcs *evmcs;
-	struct hv_partition_assist_pg **p_hv_pa_pg =
-			&to_kvm_hv(vcpu->kvm)->hv_pa_pg;
-	/*
-	 * Synthetic VM-Exit is not enabled in current code and so All
-	 * evmcs in singe VM shares same assist page.
-	 */
-	if (!*p_hv_pa_pg)
-		*p_hv_pa_pg = kzalloc(PAGE_SIZE, GFP_KERNEL_ACCOUNT);
+	hpa_t partition_assist_page = hv_get_partition_assist_page(vcpu);
 
-	if (!*p_hv_pa_pg)
+	if (partition_assist_page == INVALID_PAGE)
 		return -ENOMEM;
 
 	evmcs = (struct hv_enlightened_vmcs *)to_vmx(vcpu)->loaded_vmcs->vmcs;
 
-	evmcs->partition_assist_page =
-		__pa(*p_hv_pa_pg);
+	evmcs->partition_assist_page = partition_assist_page;
 	evmcs->hv_vm_id = (unsigned long)vcpu->kvm;
 	evmcs->hv_enlightenments_control.nested_flush_hypercall = 1;
 

arch/x86/kvm/x86.c

···
 
 void kvm_arch_free_vm(struct kvm *kvm)
 {
-	kfree(to_kvm_hv(kvm)->hv_pa_pg);
+#if IS_ENABLED(CONFIG_HYPERV)
+	kfree(kvm->arch.hv_pa_pg);
+#endif
 	__kvm_arch_free_vm(kvm);
 }