qemu: Allow system pages to <memoryBacking/>

https://bugzilla.redhat.com/show_bug.cgi?id=1173507

It occurred to me that OpenStack uses the following XML when not using
regular huge pages:

  <memoryBacking>
    <hugepages>
      <page size='4' unit='KiB'/>
    </hugepages>
  </memoryBacking>

However, since we are expecting to see huge pages only, we fail to
startup the domain with following error:

  libvirtError: internal error: Unable to find any usable hugetlbfs
  mount for 4 KiB

While regular system pages are not huge pages technically, our code is
prepared for that and if it helps OpenStack (or other management
applications) we should cope with that.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>

src/qemu/qemu_command.c

@@ -6616,6 +6616,7 @@ qemuBuildNumaArgStr(virQEMUDriverConfigPtr cfg,
     char *nodemask = NULL;
     char *mem_path = NULL;
     int ret = -1;
+    const long system_page_size = sysconf(_SC_PAGESIZE);
 
     if (virDomainNumatuneHasPerNodeBinding(def->numatune) &&
         !(virQEMUCapsGet(qemuCaps, QEMU_CAPS_OBJECT_MEMORY_RAM) ||
@@ -6626,7 +6627,8 @@ qemuBuildNumaArgStr(virQEMUDriverConfigPtr cfg,
         goto cleanup;
     }
 
-    if (def->mem.nhugepages && def->mem.hugepages[0].size &&
+    if (def->mem.nhugepages &&
+        def->mem.hugepages[0].size != system_page_size &&
         !virQEMUCapsGet(qemuCaps, QEMU_CAPS_OBJECT_MEMORY_FILE)) {
         virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
                        _("huge pages per NUMA node are not "
@@ -6720,6 +6722,13 @@ qemuBuildNumaArgStr(virQEMUDriverConfigPtr cfg,
                 hugepage = master_hugepage;
             }
 
+            if (hugepage && hugepage->size == system_page_size) {
+                /* However, if user specified to use "huge" page
+                 * of regular system page size, it's as if they
+                 * hasn't specified any huge pages at all. */
+                hugepage = NULL;
+            }
+
             if (hugepage) {
                 /* Now lets see, if the huge page we want to use is even mounted
                  * and ready to use */
@@ -7882,8 +7891,13 @@ qemuBuildCommandLine(virConnectPtr conn,
     def->mem.max_balloon = VIR_DIV_UP(def->mem.max_balloon, 1024) * 1024;
     virCommandAddArgFormat(cmd, "%llu", def->mem.max_balloon / 1024);
     if (def->mem.nhugepages && (!def->cpu || !def->cpu->ncells)) {
-        char *mem_path;
+        const long system_page_size = sysconf(_SC_PAGESIZE) / 1024;
+        char *mem_path = NULL;
 
+        if (def->mem.hugepages[0].size == system_page_size) {
+            /* There is one special case: if user specified "huge"
+             * pages of regular system pages size. */
+        } else {
             if (!cfg->nhugetlbfs) {
                 virReportError(VIR_ERR_INTERNAL_ERROR,
                                "%s", _("hugetlbfs filesystem is not mounted "
@@ -7917,9 +7931,11 @@ qemuBuildCommandLine(virConnectPtr conn,
                                                         cfg->nhugetlbfs)))
                     goto error;
             }
+        }
 
-        virCommandAddArgList(cmd, "-mem-prealloc", "-mem-path",
+        virCommandAddArg(cmd, "-mem-prealloc");
-                             mem_path, NULL);
+        if (mem_path)
+            virCommandAddArgList(cmd, "-mem-path", mem_path, NULL);
         VIR_FREE(mem_path);
     }
 

tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages6.args

@@ -0,0 +1,4 @@
+LC_ALL=C PATH=/bin HOME=/home/test USER=test LOGNAME=test QEMU_AUDIO_DRV=none \
+/usr/bin/qemu -S -M pc -m 1024 -mem-prealloc -smp 2 -nographic \
+-monitor unix:/tmp/test-monitor,server,nowait -no-acpi -boot c -usb \
+-hda /dev/HostVG/QEMUGuest1 -net none -serial none -parallel none

tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages6.xml

@@ -0,0 +1,32 @@
+<domain type='qemu'>
+  <name>SomeDummyHugepagesGuest</name>
+  <uuid>ef1bdff4-27f3-4e85-a807-5fb4d58463cc</uuid>
+  <memory unit='KiB'>1048576</memory>
+  <currentMemory unit='KiB'>1048576</currentMemory>
+  <memoryBacking>
+    <hugepages>
+      <page size='4' unit='KiB'/>
+    </hugepages>
+  </memoryBacking>
+  <vcpu placement='static'>2</vcpu>
+  <os>
+    <type arch='i686' machine='pc'>hvm</type>
+    <boot dev='hd'/>
+  </os>
+  <clock offset='utc'/>
+  <on_poweroff>destroy</on_poweroff>
+  <on_reboot>restart</on_reboot>
+  <on_crash>destroy</on_crash>
+  <devices>
+    <emulator>/usr/bin/qemu</emulator>
+    <disk type='block' device='disk'>
+      <source dev='/dev/HostVG/QEMUGuest1'/>
+      <target dev='hda' bus='ide'/>
+      <address type='drive' controller='0' bus='0' target='0' unit='0'/>
+    </disk>
+    <controller type='usb' index='0'/>
+    <controller type='ide' index='0'/>
+    <controller type='pci' index='0' model='pci-root'/>
+    <memballoon model='virtio'/>
+  </devices>
+</domain>

tests/qemuxml2argvtest.c

@@ -713,6 +713,7 @@ mymain(void)
     DO_TEST_FAILURE("hugepages-pages4", QEMU_CAPS_MEM_PATH,
             QEMU_CAPS_OBJECT_MEMORY_RAM, QEMU_CAPS_OBJECT_MEMORY_FILE);
     DO_TEST("hugepages-pages5", QEMU_CAPS_MEM_PATH);
+    DO_TEST("hugepages-pages6", NONE);
     DO_TEST("nosharepages", QEMU_CAPS_MACHINE_OPT, QEMU_CAPS_MEM_MERGE);
     DO_TEST("disk-cdrom", NONE);
     DO_TEST("disk-cdrom-network-http", QEMU_CAPS_KVM, QEMU_CAPS_DEVICE,