[PATCH v5 6/7] kvm, mem-hotplug: Unpin and remove kvm_arch->apic_access_page.

[Tang Chen <tangchen@cn.fujitsu.com>]

To make apic access page migratable, we do not pin it in memory now.
When it is migrated, we should reload its physical address for all
vmcses. But when we tried to do this, all vcpu will access
kvm_arch->apic_access_page without any locking. This is not safe.

Actually, we do not need kvm_arch->apic_access_page anymore. Since
apic access page is not pinned in memory now, we can remove
kvm_arch->apic_access_page. When we need to write its physical address
into vmcs, use gfn_to_page() to get its page struct, which will also
pin it. And unpin it after then.

Suggested-by: Gleb Natapov <gleb@kernel.org>
Signed-off-by: Tang Chen <tangchen@cn.fujitsu.com>
---
 arch/x86/include/asm/kvm_host.h |  2 +-
 arch/x86/kvm/vmx.c              | 32 +++++++++++++++++++++++++-------
 arch/x86/kvm/x86.c              | 15 +++++++++------
 3 files changed, 35 insertions(+), 14 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 514183e..70f0d2d 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -576,7 +576,7 @@ struct kvm_arch {
 	struct kvm_apic_map *apic_map;
 
 	unsigned int tss_addr;
-	struct page *apic_access_page;
+	bool apic_access_page_done;
 
 	gpa_t wall_clock;
 
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index e7704b2..058c373 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -4002,7 +4002,7 @@ static int alloc_apic_access_page(struct kvm *kvm)
 	int r = 0;
 
 	mutex_lock(&kvm->slots_lock);
-	if (kvm->arch.apic_access_page)
+	if (kvm->arch.apic_access_page_done)
 		goto out;
 	kvm_userspace_mem.slot = APIC_ACCESS_PAGE_PRIVATE_MEMSLOT;
 	kvm_userspace_mem.flags = 0;
@@ -4018,7 +4018,12 @@ static int alloc_apic_access_page(struct kvm *kvm)
 		goto out;
 	}
 
-	kvm->arch.apic_access_page = page;
+	/*
+	 * Do not pin apic access page in memory so that memory hotplug
+	 * process is able to migrate it.
+	 */
+	put_page(page);
+	kvm->arch.apic_access_page_done = true;
 out:
 	mutex_unlock(&kvm->slots_lock);
 	return r;
@@ -4536,9 +4541,16 @@ static void vmx_vcpu_reset(struct kvm_vcpu *vcpu)
 		vmcs_write32(TPR_THRESHOLD, 0);
 	}
 
-	if (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm))
-		vmcs_write64(APIC_ACCESS_ADDR,
-			     page_to_phys(vmx->vcpu.kvm->arch.apic_access_page));
+	if (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm)) {
+		struct page *page = gfn_to_page(vmx->vcpu.kvm,
+				APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
+		vmcs_write64(APIC_ACCESS_ADDR, page_to_phys(page));
+		/*
+		 * Do not pin apic access page in memory so that memory
+		 * hotplug process is able to migrate it.
+		 */
+		put_page(page);
+	}
 
 	if (vmx_vm_has_apicv(vcpu->kvm))
 		memset(&vmx->pi_desc, 0, sizeof(struct pi_desc));
@@ -7994,10 +8006,16 @@ static void prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12)
 				vmcs_write64(APIC_ACCESS_ADDR,
 				  page_to_phys(vmx->nested.apic_access_page));
 		} else if (vm_need_virtualize_apic_accesses(vmx->vcpu.kvm)) {
+			struct page *page = gfn_to_page(vmx->vcpu.kvm,
+				APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
 			exec_control |=
 				SECONDARY_EXEC_VIRTUALIZE_APIC_ACCESSES;
-			vmcs_write64(APIC_ACCESS_ADDR,
-				page_to_phys(vcpu->kvm->arch.apic_access_page));
+			vmcs_write64(APIC_ACCESS_ADDR, page_to_phys(page));
+			/*
+			 * Do not pin apic access page in memory so that memory
+			 * hotplug process is able to migrate it.
+			 */
+			put_page(page);
 		}
 
 		vmcs_write32(SECONDARY_VM_EXEC_CONTROL, exec_control);
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 96f4188..6da0b93 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -5991,15 +5991,20 @@ static void vcpu_scan_ioapic(struct kvm_vcpu *vcpu)
 
 static void vcpu_reload_apic_access_page(struct kvm_vcpu *vcpu)
 {
+	struct page *page;
+
 	/*
 	 * apic access page could be migrated. When the page is being migrated,
 	 * GUP will wait till the migrate entry is replaced with the new pte
 	 * entry pointing to the new page.
 	 */
-	vcpu->kvm->arch.apic_access_page = gfn_to_page(vcpu->kvm,
-				APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
-	kvm_x86_ops->set_apic_access_page_addr(vcpu->kvm,
-				page_to_phys(vcpu->kvm->arch.apic_access_page));
+	page = gfn_to_page(vcpu->kvm, APIC_DEFAULT_PHYS_BASE >> PAGE_SHIFT);
+	kvm_x86_ops->set_apic_access_page_addr(vcpu->kvm, page_to_phys(page));
+	/*
+	 * Do not pin apic access page in memory so that memory hotplug
+	 * process is able to migrate it.
+	 */
+	put_page(page);
 }
 
 /*
@@ -7252,8 +7257,6 @@ void kvm_arch_destroy_vm(struct kvm *kvm)
 	kfree(kvm->arch.vpic);
 	kfree(kvm->arch.vioapic);
 	kvm_free_vcpus(kvm);
-	if (kvm->arch.apic_access_page)
-		put_page(kvm->arch.apic_access_page);
 	kfree(rcu_dereference_check(kvm->arch.apic_map, 1));
 }
 
-- 
1.8.3.1