KVM: s390: pv: avoid export before import if possible

If the appropriate UV feature bit is set, there is no need to perform
an export before import.

The misc feature indicates, among other things, that importing a shared
page from a different protected VM will automatically also transfer its
ownership.
Signed-off-by: NClaudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: NNico Boehr <nrb@linux.ibm.com>
Reviewed-by: NJanosch Frank <frankja@linux.ibm.com>
Reviewed-by: NSteffen Eiden <seiden@linux.ibm.com>
Link: https://lore.kernel.org/r/20221111170632.77622-5-imbrenda@linux.ibm.com
Message-Id: <20221111170632.77622-5-imbrenda@linux.ibm.com>
Signed-off-by: NJanosch Frank <frankja@linux.ibm.com>

arch/s390/kernel/uv.c

@@ -255,6 +255,13 @@ static int make_secure_pte(pte_t *ptep, unsigned long addr,
  */
 static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_struct *mm)
 {
+	/*
+	 * The misc feature indicates, among other things, that importing a
+	 * shared page from a different protected VM will automatically also
+	 * transfer its ownership.
+	 */
+	if (test_bit_inv(BIT_UV_FEAT_MISC, &uv_info.uv_feature_indications))
+		return false;
 	if (uvcb->cmd == UVC_CMD_UNPIN_PAGE_SHARED)
 		return false;
 	return atomic_read(&mm->context.protected_count) > 1;