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: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Steffen 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: Janosch Frank <frankja@linux.ibm.com>

diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
index f9810d2a267c68b37bcb940e59d3896444ef024e..9f18a4af9c13198468602bc56f22342b5e4a373b 100644 (file)
--- a/arch/s390/kernel/uv.c
+++ b/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;