KVM: nVMX: Preserve last USE_MSR_BITMAPS when preparing vmcs02

KVM dynamically toggles the CPU_BASED_USE_MSR_BITMAPS execution control
for nested guests based on whether or not both L0 and L1 want to pass
through the same MSRs to L2.  Preserve the last used value from vmcs02
so as to avoid multiple VMWRITEs to (re)set/(re)clear the bit on nested
VM-Entry.
Signed-off-by: Sean Christopherson <sean.j.christopherson@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

arch/x86/kvm/vmx/nested.c

@@ -2047,8 +2047,18 @@ static void prepare_vmcs02_early(struct vcpu_vmx *vmx, struct vmcs12 *vmcs12)
 	 * A vmexit (to either L1 hypervisor or L0 userspace) is always needed
 	 * for I/O port accesses.
 	 */
-	exec_control &= ~CPU_BASED_USE_IO_BITMAPS;
 	exec_control |= CPU_BASED_UNCOND_IO_EXITING;
+	exec_control &= ~CPU_BASED_USE_IO_BITMAPS;
+
+	/*
+	 * This bit will be computed in nested_get_vmcs12_pages, because
+	 * we do not have access to L1's MSR bitmap yet.  For now, keep
+	 * the same bit as before, hoping to avoid multiple VMWRITEs that
+	 * only set/clear this bit.
+	 */
+	exec_control &= ~CPU_BASED_USE_MSR_BITMAPS;
+	exec_control |= exec_controls_get(vmx) & CPU_BASED_USE_MSR_BITMAPS;
+
 	exec_controls_set(vmx, exec_control);
 
 	/*