qemuBuildMemPathStr: Produce -mem-path more frequently

https://bugzilla.redhat.com/show_bug.cgi?id=1622455

If a domain is configured to use <source type='file'/> under
<memoryBacking/> we have to honour that setting and produce
-mem-path on the command line. We are not doing so if domain has
no guest NUMA nodes nor hugepages.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: John Ferlan <jferlan@redhat.com>

src/qemu/qemu_command.c

@@ -7531,21 +7531,22 @@ qemuBuildMemPathStr(virQEMUDriverConfigPtr cfg,
     const long system_page_size = virGetSystemPageSizeKB();
     char *mem_path = NULL;
 
-    /*
-     *  No-op if hugepages were not requested.
-     */
-    if (!def->mem.nhugepages)
+    /* There are two cases where we want to put -mem-path onto
+     * the command line: First one is when there are no guest
+     * NUMA nodes and hugepages are configured. The second one is
+     * if user requested file allocation. */
+    if (def->mem.nhugepages &&
+        def->mem.hugepages[0].size != system_page_size) {
+        if (qemuGetDomainHupageMemPath(def, cfg,
+                                       def->mem.hugepages[0].size,
+                                       &mem_path) < 0)
+            return -1;
+    } else if (def->mem.source == VIR_DOMAIN_MEMORY_SOURCE_FILE) {
+        if (qemuGetMemoryBackingPath(def, cfg, "ram", &mem_path) < 0)
+            return -1;
+    } else {
         return 0;
-
-    /* There is one special case: if user specified "huge"
-     * pages of regular system pages size.
-     * And there is nothing to do in this case.
-     */
-    if (def->mem.hugepages[0].size == system_page_size)
-        return 0;
-
-    if (qemuGetDomainHupageMemPath(def, cfg, def->mem.hugepages[0].size, &mem_path) < 0)
-        return -1;
+    }
 
     if (def->mem.allocation != VIR_DOMAIN_MEMORY_ALLOCATION_IMMEDIATE)
         virCommandAddArgList(cmd, "-mem-prealloc", NULL);

tests/qemuxml2argvdata/fd-memory-no-numa-topology.args

@@ -10,6 +10,7 @@ QEMU_AUDIO_DRV=none \
 -machine pc-i440fx-wily,accel=kvm,usb=off,dump-guest-core=off \
 -m 14336 \
 -mem-prealloc \
+-mem-path /var/lib/libvirt/qemu/ram/libvirt/qemu/-1-instance-00000092/ram \
 -smp 8,sockets=8,cores=1,threads=1 \
 -uuid 126f2720-6f8e-45ab-a886-ec9277079a67 \
 -display none \