[libvirt] [PATCH] qemu: Only use memory-backend-file with NUMA if needed

[Martin Kletzander]

If this reminds you of a commit message from around a year ago, it's
41c2aa729f0af084ede95ee9a06219a2dd5fb5df and yes, we're dealing with
"the same thing" again.  Or f309db1f4d51009bad0d32e12efc75530b66836b and
it's similar.

There is a logic in place that if there is no real need for
memory-backend-file, qemuBuildMemoryBackendStr() returns 0.  However
that wasn't the case with hugepage backing.  The reason for that was
that we abused the 'pagesize' variable for storing that information, but
we should rather have a separate one that specifies whether we really
need the new object for hugepage backing.  And that variable should be
set only if this particular NUMA cell needs special treatment WRT
hugepages.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1372153

Signed-off-by: Martin Kletzander <mkletzan at redhat.com>
---

Notes:
    This fixes migration from older libvirts.  By "older", I mean
    pre-(circa-)1.2.7, also in some cases pre-1.2.11, in some other cases
    pre-v1.2.20.  It's pretty messy.  It could be back-ported as far as it's
    easy to do.

 src/qemu/qemu_command.c                                   |  8 +++++---
 tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages2.args | 10 ++++------
 2 files changed, 9 insertions(+), 9 deletions(-)

diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 1ac0fa116310..316aced5124a 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -3202,6 +3202,7 @@ qemuBuildMemoryBackendStr(unsigned long long size,
     int ret = -1;
     virJSONValuePtr props = NULL;
     bool nodeSpecified = virDomainNumatuneNodeSpecified(def->numa, guestNode);
+    bool needHugepage = !!pagesize;

     *backendProps = NULL;
     *backendType = NULL;
@@ -3224,10 +3225,10 @@ qemuBuildMemoryBackendStr(unsigned long long size,
         mode = VIR_DOMAIN_NUMATUNE_MEM_STRICT;

     if (pagesize == 0) {
+        bool thisHugepage = false;
+
         /* Find the huge page size we want to use */
         for (i = 0; i < def->mem.nhugepages; i++) {
-            bool thisHugepage = false;
-
             hugepage = &def->mem.hugepages[i];

             if (!hugepage->nodemask) {
@@ -3249,6 +3250,7 @@ qemuBuildMemoryBackendStr(unsigned long long size,

             if (thisHugepage) {
                 /* Hooray, we've found the page size */
+                needHugepage = true;
                 break;
             }
         }
@@ -3335,7 +3337,7 @@ qemuBuildMemoryBackendStr(unsigned long long size,
     }

     /* If none of the following is requested... */
-    if (!pagesize && !userNodeset && !memAccess && !nodeSpecified && !force) {
+    if (!needHugepage && !userNodeset && !memAccess && !nodeSpecified && !force) {
         /* report back that using the new backend is not necessary
          * to achieve the desired configuration */
         ret = 1;
diff --git a/tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages2.args b/tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages2.args
index 447bb52d00b7..a37f03d0d2ed 100644
--- a/tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages2.args
+++ b/tests/qemuxml2argvdata/qemuxml2argv-hugepages-pages2.args
@@ -10,12 +10,10 @@ QEMU_AUDIO_DRV=none \
 -M pc \
 -m 1024 \
 -smp 2,sockets=2,cores=1,threads=1 \
--object memory-backend-file,id=ram-node0,prealloc=yes,\
-mem-path=/dev/hugepages2M/libvirt/qemu,size=268435456 \
--numa node,nodeid=0,cpus=0,memdev=ram-node0 \
--object memory-backend-file,id=ram-node1,prealloc=yes,\
-mem-path=/dev/hugepages2M/libvirt/qemu,size=805306368 \
--numa node,nodeid=1,cpus=1,memdev=ram-node1 \
+-mem-prealloc \
+-mem-path /dev/hugepages2M/libvirt/qemu \
+-numa node,nodeid=0,cpus=0,mem=256 \
+-numa node,nodeid=1,cpus=1,mem=768 \
 -uuid ef1bdff4-27f3-4e85-a807-5fb4d58463cc \
 -nographic \
 -nodefaults \
-- 
2.10.0