KVM: VMX: Fix commit which broke PML

I found PML was broken since below commit:

commit feda805fe7c4ed9cf78158e73b1218752e3b4314
Author: Xiao Guangrong <[e-mail]>
Date: Wed Sep 9 14:05:55 2015 +0800

KVM: VMX: unify SECONDARY_VM_EXEC_CONTROL update

Unify the update in vmx_cpuid_update()

Signed-off-by: Xiao Guangrong <[e-mail]>
[Rewrite to use vmcs_set_secondary_exec_control. - Paolo]
Signed-off-by: Paolo Bonzini <[e-mail]>

The reason is in above commit vmx_cpuid_update calls vmx_secondary_exec_control,
in which currently SECONDARY_EXEC_ENABLE_PML bit is cleared unconditionally (as
PML is enabled in creating vcpu). Therefore if vcpu_cpuid_update is called after
vcpu is created, PML will be disabled unexpectedly while log-dirty code still
thinks PML is used.

Fix this by clearing SECONDARY_EXEC_ENABLE_PML in vmx_secondary_exec_control
only when PML is not supported or not enabled (!enable_pml). This is more
reasonable as PML is currently either always enabled or disabled. With this
explicit updating SECONDARY_EXEC_ENABLE_PML in vmx_enable{disable}_pml is not
needed so also rename vmx_enable{disable}_pml to vmx_create{destroy}_pml_buffer.

Fixes: feda805fe7c4ed9cf78158e73b1218752e3b4314
Signed-off-by: Kai Huang <[e-mail]>
[While at it, change a wrong ASSERT to an "if". The condition can happen
if creating the VCPU fails with ENOMEM. - Paolo]
Signed-off-by: Paolo Bonzini <[e-mail]>

arch/x86/kvm/vmx.c

···
 	   a current VMCS12
 	*/
 	exec_control &= ~SECONDARY_EXEC_SHADOW_VMCS;
-	/* PML is enabled/disabled in creating/destorying vcpu */
-	exec_control &= ~SECONDARY_EXEC_ENABLE_PML;
+
+	if (!enable_pml)
+		exec_control &= ~SECONDARY_EXEC_ENABLE_PML;
 
 	/* Currently, we allow L1 guest to directly run pcommit instruction. */
 	exec_control &= ~SECONDARY_EXEC_PCOMMIT;
···
 	*info2 = vmcs_read32(VM_EXIT_INTR_INFO);
 }
 
-static int vmx_enable_pml(struct vcpu_vmx *vmx)
+static int vmx_create_pml_buffer(struct vcpu_vmx *vmx)
 {
 	struct page *pml_pg;
 
···
 	vmcs_write64(PML_ADDRESS, page_to_phys(vmx->pml_pg));
 	vmcs_write16(GUEST_PML_INDEX, PML_ENTITY_NUM - 1);
 
-	vmcs_set_bits(SECONDARY_VM_EXEC_CONTROL, SECONDARY_EXEC_ENABLE_PML);
-
 	return 0;
 }
 
-static void vmx_disable_pml(struct vcpu_vmx *vmx)
+static void vmx_destroy_pml_buffer(struct vcpu_vmx *vmx)
 {
-	ASSERT(vmx->pml_pg);
-	__free_page(vmx->pml_pg);
-	vmx->pml_pg = NULL;
-
-	vmcs_clear_bits(SECONDARY_VM_EXEC_CONTROL, SECONDARY_EXEC_ENABLE_PML);
+	if (vmx->pml_pg) {
+		__free_page(vmx->pml_pg);
+		vmx->pml_pg = NULL;
+	}
 }
 
 static void vmx_flush_pml_buffer(struct kvm_vcpu *vcpu)
···
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
 
 	if (enable_pml)
-		vmx_disable_pml(vmx);
+		vmx_destroy_pml_buffer(vmx);
 	free_vpid(vmx->vpid);
 	leave_guest_mode(vcpu);
 	vmx_load_vmcs01(vcpu);
···
 	 * for the guest, etc.
 	 */
 	if (enable_pml) {
-		err = vmx_enable_pml(vmx);
+		err = vmx_create_pml_buffer(vmx);
 		if (err)
 			goto free_vmcs;
 	}