[Qemu-devel] [PATCH V3] spapr: Fix stale HTAB during live migration

[Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>]

If a guest reboots during a running migration, changes to the
hash page table are not necessarily updated on the destination.
Opening a new file descriptor to the HTAB forces the migration
handler to resend the entire table.

Signed-off-by: Samuel Mendoza-Jonas <sam.mj@au1.ibm.com>
---
Changes in v3: Pointed out by David, htab_save_iterate could
	potentially try to read before htab_fd is open again.
	Leave opening the fd to the functions trying to read.
Changes in v2: Forgot check on kvmppc_get_htab_fd return value
 hw/ppc/spapr.c         | 25 +++++++++++++++++++++++++
 include/hw/ppc/spapr.h |  1 +
 2 files changed, 26 insertions(+)

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 3a6d26d..5b41318 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -997,6 +997,10 @@ static void spapr_reset_htab(sPAPREnvironment *spapr)
         /* Kernel handles htab, we don't need to allocate one */
         spapr->htab_shift = shift;
         kvmppc_kern_htab = true;
+
+        /* Check if we are overlapping a migration */
+        if (spapr->htab_fd > 0)
+            spapr->need_reset = true;
     } else {
         if (!spapr->htab) {
             /* Allocate an htab if we don't yet have one */
@@ -1156,6 +1160,7 @@ static int htab_save_setup(QEMUFile *f, void *opaque)
     } else {
         assert(kvm_enabled());
 
+        spapr->need_reset = false;
         spapr->htab_fd = kvmppc_get_htab_fd(false);
         if (spapr->htab_fd < 0) {
             fprintf(stderr, "Unable to open fd for reading hash table from KVM: %s\n",
@@ -1309,6 +1314,16 @@ static int htab_save_iterate(QEMUFile *f, void *opaque)
     if (!spapr->htab) {
         assert(kvm_enabled());
 
+        if (atomic_cmpxchg(&spapr->need_reset, true, false) == true) {
+            close(spapr->htab_fd);
+            spapr->htab_fd = kvmppc_get_htab_fd(false);
+            if (spapr->htab_fd < 0) {
+                fprintf(stderr, "Unable to open fd for reading hash table from KVM: %s\n",
+                        strerror(errno));
+                return -1;
+            }
+        }
+
         rc = kvmppc_save_htab(f, spapr->htab_fd,
                               MAX_KVM_BUF_SIZE, MAX_ITERATION_NS);
         if (rc < 0) {
@@ -1340,6 +1355,16 @@ static int htab_save_complete(QEMUFile *f, void *opaque)
 
         assert(kvm_enabled());
 
+        if (atomic_cmpxchg(&spapr->need_reset, true, false) == true) {
+            close(spapr->htab_fd);
+            spapr->htab_fd = kvmppc_get_htab_fd(false);
+            if (spapr->htab_fd < 0) {
+                fprintf(stderr, "Unable to open fd for reading hash table from KVM: %s\n",
+                        strerror(errno));
+                return -1;
+            }
+        }
+
         rc = kvmppc_save_htab(f, spapr->htab_fd, MAX_KVM_BUF_SIZE, -1);
         if (rc < 0) {
             return rc;
diff --git a/include/hw/ppc/spapr.h b/include/hw/ppc/spapr.h
index 0c2e3c5..9ab9827 100644
--- a/include/hw/ppc/spapr.h
+++ b/include/hw/ppc/spapr.h
@@ -71,6 +71,7 @@ typedef struct sPAPREnvironment {
     int htab_save_index;
     bool htab_first_pass;
     int htab_fd;
+    bool need_reset;
 
     /* state for Dynamic Reconfiguration Connectors */
     sPAPRDrcEntry drc_table[SPAPR_DRC_TABLE_SIZE];
-- 
1.9.3