[PATCH v10 05/16] s390x: protvirt: Inhibit balloon when switching to protected mode

[Janosch Frank]

Ballooning in protected VMs can only be done when the guest shares the
pages it gives to the host. If pages are not shared, the integrity
checks will fail once those pages have been altered and are given back
to the guest.

As we currently do not yet have a solution for this we will continue
like this:

1. We block ballooning now in QEMU (with this patch).

2. Later we will provide a change to virtio that removes the blocker
and adds VIRTIO_F_IOMMU_PLATFORM automatically by QEMU when doing the
protvirt switch. This is OK, as the balloon driver in Linux (the only
supported guest) will refuse to work with the IOMMU_PLATFORM feature
bit set.

3. Later, we can fix the guest balloon driver to accept the IOMMU
feature bit and correctly exercise sharing and unsharing of balloon
pages.

Signed-off-by: Janosch Frank <address@hidden>
Reviewed-by: David Hildenbrand <address@hidden>
Reviewed-by: Christian Borntraeger <address@hidden>
Reviewed-by: Claudio Imbrenda <address@hidden>
---
 hw/s390x/s390-virtio-ccw.c | 11 +++++++++++
 1 file changed, 11 insertions(+)

diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 8013c82236a36724..3cf19c99f3468b7d 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -42,6 +42,7 @@
 #include "hw/qdev-properties.h"
 #include "hw/s390x/tod.h"
 #include "sysemu/sysemu.h"
+#include "sysemu/balloon.h"
 #include "hw/s390x/pv.h"
 #include <linux/kvm.h>
 #include "migration/blocker.h"
@@ -328,6 +329,7 @@ static void s390_machine_unprotect(S390CcwMachineState *ms)
     ms->pv = false;
     migrate_del_blocker(pv_mig_blocker);
     error_free_or_abort(&pv_mig_blocker);
+    qemu_balloon_inhibit(false);
 }
 
 static int s390_machine_protect(S390CcwMachineState *ms)
@@ -335,10 +337,18 @@ static int s390_machine_protect(S390CcwMachineState *ms)
     Error *local_err = NULL;
     int rc;
 
+   /*
+    * Ballooning on protected VMs needs support in the guest for
+    * sharing and unsharing balloon pages. Block ballooning for
+    * now, until we have a solution to make at least Linux guests
+    * either support it or fail gracefully.
+    */
+    qemu_balloon_inhibit(true);
     error_setg(&pv_mig_blocker,
                "protected VMs are currently not migrateable.");
     rc = migrate_add_blocker(pv_mig_blocker, &local_err);
     if (rc) {
+        qemu_balloon_inhibit(false);
         error_report_err(local_err);
         error_free_or_abort(&pv_mig_blocker);
         return rc;
@@ -347,6 +357,7 @@ static int s390_machine_protect(S390CcwMachineState *ms)
     /* Create SE VM */
     rc = s390_pv_vm_enable();
     if (rc) {
+        qemu_balloon_inhibit(false);
         error_report_err(local_err);
         migrate_del_blocker(pv_mig_blocker);
         error_free_or_abort(&pv_mig_blocker);
-- 
2.25.1