[PATCH v2] conf: Introduce memory allocation threads

[Michal Privoznik]

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 at redhat.com>
---
 docs/formatdomain.rst                        |  8 +++++---
 src/conf/domain_conf.c                       | 15 ++++++++++++++-
 src/conf/domain_conf.h                       |  1 +
 src/conf/schemas/domaincommon.rng            | 19 +++++++++++++------
 tests/qemuxml2argvdata/memfd-memory-numa.xml |  2 +-
 5 files changed, 34 insertions(+), 11 deletions(-)

diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
index d188de4858..e492532004 100644
--- a/docs/formatdomain.rst
+++ b/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
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 153954a0b0..731139f80f 100644
--- a/src/conf/domain_conf.c
+++ b/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");
 
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index b69abfa270..49c964e6e1 100644
--- a/src/conf/domain_conf.h
+++ b/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;
 };
diff --git a/src/conf/schemas/domaincommon.rng b/src/conf/schemas/domaincommon.rng
index 9c1b64a644..34bccee2f5 100644
--- a/src/conf/schemas/domaincommon.rng
+++ b/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>
diff --git a/tests/qemuxml2argvdata/memfd-memory-numa.xml b/tests/qemuxml2argvdata/memfd-memory-numa.xml
index 1ebcee8939..1ac87e3aef 100644
--- a/tests/qemuxml2argvdata/memfd-memory-numa.xml
+++ b/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>
-- 
2.34.1