[libvirt] [PATCH 1/2] numa: deprecate 'mem' parameter of '-numa node' option

[Igor Mammedov]

The parameter allows to configure fake NUMA topology where guest
VM simulates NUMA topology but not actually getting a performance
benefits from it. The same or better results could be achieved
using 'memdev' parameter. In light of that any VM that uses NUMA
to get its benefits should use 'memdev' and to allow transition
initial RAM to device based model, deprecate 'mem' parameter as
its ad-hoc partitioning of initial RAM MemoryRegion can't be
translated to memdev based backend transparently to users and in
compatible manner (migration wise).

That will also allow to clean up a bit our numa code, leaving only
'memdev' impl. in place and several boards that use node_mem
to generate FDT/ACPI description from it.

Signed-off-by: Igor Mammedov <imammedo at redhat.com>
---
 numa.c               |  2 ++
 qemu-deprecated.texi | 14 ++++++++++++++
 2 files changed, 16 insertions(+)

diff --git a/numa.c b/numa.c
index 3875e1e..2205773 100644
--- a/numa.c
+++ b/numa.c
@@ -121,6 +121,8 @@ static void parse_numa_node(MachineState *ms, NumaNodeOptions *node,
 
     if (node->has_mem) {
         numa_info[nodenr].node_mem = node->mem;
+        warn_report("Parameter -numa node,mem is deprecated,"
+                    " use -numa node,memdev instead");
     }
     if (node->has_memdev) {
         Object *o;
diff --git a/qemu-deprecated.texi b/qemu-deprecated.texi
index 45c5795..73f99d4 100644
--- a/qemu-deprecated.texi
+++ b/qemu-deprecated.texi
@@ -60,6 +60,20 @@ Support for invalid topologies will be removed, the user must ensure
 topologies described with -smp include all possible cpus, i.e.
   @math{@var{sockets} * @var{cores} * @var{threads} = @var{maxcpus}}.
 
+ at subsection -numa node,mem=@var{size} (since 4.0)
+
+The parameter @option{mem} of @option{-numa node} is used to assign a part of
+guest RAM to a NUMA node. But when using it, it's impossible to manage specified
+size on the host side (like bind it to a host node, setting bind policy, ...),
+so guest end-ups with the fake NUMA configuration with suboptiomal performance.
+However since 2014 there is an alternative way to assign RAM to a NUMA node
+using parameter @option{memdev}, which does the same as @option{mem} and has
+an ability to actualy manage node RAM on the host side. Use parameter
+ at option{memdev} with @var{memory-backend-ram} backend as an replacement for
+parameter @option{mem} to achieve the same fake NUMA effect or a properly
+configured @var{memory-backend-file} backend to actually benefit from NUMA
+configuration.
+
 @section QEMU Machine Protocol (QMP) commands
 
 @subsection block-dirty-bitmap-add "autoload" parameter (since 2.12.0)
-- 
2.7.4