conf: Introduce dynamicMemslots attribute for virtio-mem

Introduced in v8.2.0-rc0~74^2~2, QEMU now allows setting
.dynamic-memslots attribute for virtio-mem-pci devices. When
turned on, it allows memory exposed to guest to be split into
multiple memslots and thus smaller memory footprint (see the
original commit for detailed explanation).

Therefore, introduce new <target/> attribute which will control
that QEMU knob.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

docs/formatdomain.rst

@@ -8443,6 +8443,19 @@ Example: usage of the memory devices
    The ``node`` subelement configures the guest NUMA node to attach the memory
    to. The element shall be used only if the guest has NUMA nodes configured.
 
+   For ``virtio-mem`` optional attribute ``dynamicMemslots`` can be specified
+   (accepted values "yes"/"no") which allows hypervisor to spread memory into
+   multiple memory slots (allocate them dynamically based on the amount of
+   memory exposed to the guest), resulting in smaller memory footprint. But be
+   aware this may affect vhost-user devices. When enabled, older vhost-user
+   device implementations (such as virtiofs) may refuse to initialize resulting
+   in failed domain startup or device hotplug. When only modern vhost-user
+   based devices will be used or when no vhost-user devices are expected to be
+   used it's beneficial to enable this feature. The current default is
+   hypervisor dependant (for QEMU is "no"). If the default changes and you are
+   having difficulties with vhost-user devices, try toggling this to "no".
+   :since:`Since 10.1.0 and QEMU 8.2.0`
+
    The following optional elements may be used:
 
    ``label``

src/conf/domain_conf.c

@@ -13558,6 +13558,10 @@ virDomainMemoryTargetDefParseXML(xmlNodePtr node,
                                  &def->target.virtio_mem.requestedsize, false, false) < 0)
             return -1;
 
+        if (virXMLPropTristateBool(node, "dynamicMemslots", VIR_XML_PROP_NONE,
+                                   &def->target.virtio_mem.dynamicMemslots) < 0)
+            return -1;
+
         addrNode = virXPathNode("./address", ctxt);
         addr = &def->target.virtio_mem.address;
         break;
@@ -21232,6 +21236,12 @@ virDomainMemoryDefCheckABIStability(virDomainMemoryDef *src,
                            src->target.virtio_mem.address);
             return false;
         }
+
+        if (src->target.virtio_mem.dynamicMemslots != dst->target.virtio_mem.dynamicMemslots) {
+            virReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                           _("Target memory device 'dynamicMemslots' property doesn't match source memory device"));
+            return false;
+        }
         break;
 
     case VIR_DOMAIN_MEMORY_MODEL_DIMM:
@@ -25448,6 +25458,7 @@ virDomainMemoryTargetDefFormat(virBuffer *buf,
                                unsigned int flags)
 {
     g_auto(virBuffer) childBuf = VIR_BUFFER_INIT_CHILD(buf);
+    g_auto(virBuffer) attrBuf = VIR_BUFFER_INITIALIZER;
 
     virBufferAsprintf(&childBuf, "<size unit='KiB'>%llu</size>\n", def->size);
     if (def->targetNode >= 0)
@@ -25487,6 +25498,11 @@ virDomainMemoryTargetDefFormat(virBuffer *buf,
         if (def->target.virtio_mem.address)
             virBufferAsprintf(&childBuf, "<address base='0x%llx'/>\n",
                               def->target.virtio_mem.address);
+
+        if (def->target.virtio_mem.dynamicMemslots) {
+            virBufferAsprintf(&attrBuf, " dynamicMemslots='%s'",
+                              virTristateBoolTypeToString(def->target.virtio_mem.dynamicMemslots));
+        }
         break;
 
     case VIR_DOMAIN_MEMORY_MODEL_SGX_EPC:
@@ -25496,7 +25512,7 @@ virDomainMemoryTargetDefFormat(virBuffer *buf,
         break;
     }
 
-    virXMLFormatElement(buf, "target", NULL, &childBuf);
+    virXMLFormatElement(buf, "target", &attrBuf, &childBuf);
 }
 
 static int

src/conf/domain_conf.h

@@ -2676,6 +2676,7 @@ struct _virDomainMemoryDef {
             unsigned long long currentsize; /* kibibytes, valid for an active
                                                domain only and parsed */
             unsigned long long address; /* address where memory is mapped */
+            virTristateBool dynamicMemslots;
         } virtio_mem;
         struct {
         } sgx_epc;

src/conf/schemas/domaincommon.rng

@@ -7268,6 +7268,11 @@
 
   <define name="memorydev-target">
     <element name="target">
+      <optional>
+        <attribute name="dynamicMemslots">
+          <ref name="virYesNo"/>
+        </attribute>
+      </optional>
       <interleave>
         <element name="size">
           <ref name="scaledInteger"/>

tests/qemuxmlconfdata/memory-hotplug-virtio-mem.xml

@@ -60,7 +60,7 @@
         <nodemask>1-3</nodemask>
         <pagesize unit='KiB'>2048</pagesize>
       </source>
-      <target>
+      <target dynamicMemslots='yes'>
         <size unit='KiB'>2097152</size>
         <node>0</node>
         <block unit='KiB'>2048</block>