qemu: Don't prealloc mem for real NVDIMMs

Currently, we configure QEMU to prealloc memory almost by
default. Well, by default for NVDIMMs, hugepages and if user
asked us to (via memoryBacking <allocation mode="immediate"/>).

However, when guest's NVDIMM is backed by real life NVDIMM this
approach is not the best. In this case users should put <pmem/>
into the <memory/> device <source/>, like this:

  <memory model='nvdimm' access='shared'>
    <source>
      <path>/dev/pmem0</path>
      <pmem/>
    </source>
  </memory>

Instructing QEMU to do prealloc in this case means that each
page of the NVDIMM is "touched" (the first byte is read and
written back - see QEMU commit v2.9.0-rc1~26^2) which cripples
device wear.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1894053
Signed-off-by: Michal Privoznik <mprivozn@redhat.com>
Reviewed-by: Daniel Henrique Barboza <danielhb413@gmail.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>

src/qemu/qemu_command.c

@@ -3085,7 +3085,10 @@ qemuBuildMemoryBackendProps(virJSONValuePtr *backendProps,
 
         if (mem->nvdimmPath) {
             memPath = g_strdup(mem->nvdimmPath);
-            prealloc = true;
+            /* If the NVDIMM is a real device then there's nothing to prealloc.
+             * If anything, we would be only wearing off the device. */
+            if (!mem->nvdimmPmem)
+                prealloc = true;
         } else if (useHugepage) {
             if (qemuGetDomainHupageMemPath(priv->driver, def, pagesize, &memPath) < 0)
                 return -1;

tests/qemuxml2argvdata/memory-hotplug-nvdimm-pmem.x86_64-latest.args

@@ -20,7 +20,7 @@ file=/tmp/lib/domain--1-QEMUGuest1/master-key.aes \
 -object memory-backend-ram,id=ram-node0,size=224395264 \
 -numa node,nodeid=0,cpus=0-1,memdev=ram-node0 \
 -object memory-backend-file,id=memnvdimm0,mem-path=/tmp/nvdimm,share=no,\
-prealloc=yes,size=536870912,pmem=yes \
+size=536870912,pmem=yes \
 -device nvdimm,node=0,memdev=memnvdimm0,id=nvdimm0,slot=0 \
 -uuid c7a5fdbd-edaf-9455-926a-d65c16db1809 \
 -display none \