conf: Introduce memory allocation threads

Since its v5.0.0 release QEMU is capable of specifying number of
threads used to allocate memory. It defaults to 1, which may be
too low for humongous guests with gigantic pages.

In general, on QEMU cmd line level it is possible to use
different number of threads per each memory-backend-* object, in
practical terms it's not useful. Therefore, use <memoryBacking/>
to set guest wide value and let all memory devices 'inherit' it,
silently. IOW, don't introduce per device knob because that would
only complicate things for a little or no benefit.

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Martin Kletzander <mkletzan@redhat.com>

docs/formatdomain.rst

@@ -1004,7 +1004,7 @@ Memory Backing
        <locked/>
        <source type="file|anonymous|memfd"/>
        <access mode="shared|private"/>
-       <allocation mode="immediate|ondemand"/>
+       <allocation mode="immediate|ondemand" threads='8'/>
        <discard/>
      </memoryBacking>
      ...
@@ -1053,8 +1053,10 @@ influence how virtual memory pages are backed by host pages.
    Using the ``mode`` attribute, specify if the memory is to be "shared" or
    "private". This can be overridden per numa node by ``memAccess``.
 ``allocation``
-   Using the ``mode`` attribute, specify when to allocate the memory by
-   supplying either "immediate" or "ondemand".
+   Using the optional ``mode`` attribute, specify when to allocate the memory by
+   supplying either "immediate" or "ondemand". :since:`Since 8.2.0` it is
+   possible to set the number of threads that hypervisor uses to allocate
+   memory via ``threads`` attribute.
 ``discard``
    When set and supported by hypervisor the memory content is discarded just
    before guest shuts down (or when DIMM module is unplugged). Please note that

src/conf/domain_conf.c

@@ -18915,6 +18915,13 @@ virDomainDefParseMemory(virDomainDef *def,
         VIR_FREE(tmp);
     }
 
+    if (virXPathUInt("string(./memoryBacking/allocation/@threads)",
+                     ctxt, &def->mem.allocation_threads) == -2) {
+        virReportError(VIR_ERR_XML_ERROR, "%s",
+                       _("Failed to parse memory allocation threads"));
+        return -1;
+    }
+
     if (virXPathNode("./memoryBacking/hugepages", ctxt)) {
         /* hugepages will be used */
         if ((n = virXPathNodeSet("./memoryBacking/hugepages/page", ctxt, &nodes)) < 0) {
@@ -27465,6 +27472,7 @@ virDomainMemorybackingFormat(virBuffer *buf,
                              const virDomainMemtune *mem)
 {
     g_auto(virBuffer) childBuf = VIR_BUFFER_INIT_CHILD(buf);
+    g_auto(virBuffer) allocAttrBuf = VIR_BUFFER_INITIALIZER;
 
     if (mem->nhugepages)
         virDomainHugepagesFormat(&childBuf, mem->hugepages, mem->nhugepages);
@@ -27479,8 +27487,13 @@ virDomainMemorybackingFormat(virBuffer *buf,
         virBufferAsprintf(&childBuf, "<access mode='%s'/>\n",
                           virDomainMemoryAccessTypeToString(mem->access));
     if (mem->allocation)
-        virBufferAsprintf(&childBuf, "<allocation mode='%s'/>\n",
+        virBufferAsprintf(&allocAttrBuf, " mode='%s'",
                           virDomainMemoryAllocationTypeToString(mem->allocation));
+    if (mem->allocation_threads > 0)
+        virBufferAsprintf(&allocAttrBuf, " threads='%u'", mem->allocation_threads);
+
+    virXMLFormatElement(&childBuf, "allocation", &allocAttrBuf, NULL);
+
     if (mem->discard)
         virBufferAddLit(&childBuf, "<discard/>\n");
 

src/conf/domain_conf.h

@@ -2702,6 +2702,7 @@ struct _virDomainMemtune {
     int source; /* enum virDomainMemorySource */
     int access; /* enum virDomainMemoryAccess */
     int allocation; /* enum virDomainMemoryAllocation */
+    unsigned int allocation_threads;
 
     virTristateBool discard;
 };

src/conf/schemas/domaincommon.rng

@@ -740,12 +740,19 @@
             </optional>
             <optional>
               <element name="allocation">
-                <attribute name="mode">
-                  <choice>
-                    <value>immediate</value>
-                    <value>ondemand</value>
-                  </choice>
-                </attribute>
+                <optional>
+                  <attribute name="mode">
+                    <choice>
+                      <value>immediate</value>
+                      <value>ondemand</value>
+                    </choice>
+                  </attribute>
+                </optional>
+                <optional>
+                  <attribute name="threads">
+                    <ref name="unsignedInt"/>
+                  </attribute>
+                </optional>
               </element>
             </optional>
             <optional>

tests/qemuxml2argvdata/memfd-memory-numa.xml

@@ -10,7 +10,7 @@
     </hugepages>
     <source type='memfd'/>
     <access mode='shared'/>
-    <allocation mode='immediate'/>
+    <allocation mode='immediate' threads='8'/>
   </memoryBacking>
   <vcpu placement='static'>8</vcpu>
   <numatune>