qemu-options: Rewrite -numa documentation

Rewrite the -numa documentation to clarify what exactly it does.

Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>
Message-Id: <20170123180632.28942-3-ehabkost@redhat.com>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Signed-off-by: Eduardo Habkost <ehabkost@redhat.com>

qemu-options.hx

@@ -144,16 +144,34 @@ STEXI
 @item -numa node[,mem=@var{size}][,cpus=@var{firstcpu}[-@var{lastcpu}]][,nodeid=@var{node}]
 @itemx -numa node[,memdev=@var{id}][,cpus=@var{firstcpu}[-@var{lastcpu}]][,nodeid=@var{node}]
 @findex -numa
-Simulate a multi node NUMA system. If @samp{mem}, @samp{memdev}
-and @samp{cpus} are omitted, resources are split equally. Also, note
-that the -@option{numa} option doesn't allocate any of the specified
-resources. That is, it just assigns existing resources to NUMA nodes. This
-means that one still has to use the @option{-m}, @option{-smp} options
-to allocate RAM and VCPUs respectively, and possibly @option{-object}
-to specify the memory backend for the @samp{memdev} suboption.
-
-@samp{mem} and @samp{memdev} are mutually exclusive.  Furthermore, if one
-node uses @samp{memdev}, all of them have to use it.
+Define a NUMA node and assign RAM and VCPUs to it.
+
+@var{firstcpu} and @var{lastcpu} are CPU indexes. Each
+@samp{cpus} option represent a contiguous range of CPU indexes
+(or a single VCPU if @var{lastcpu} is omitted). A non-contiguous
+set of VCPUs can be represented by providing multiple @samp{cpus}
+options. If @samp{cpus} is omitted on all nodes, VCPUs are automatically
+split between them.
+
+For example, the following option assigns VCPUs 0, 1, 2 and 5 to
+a NUMA node:
+@example
+-numa node,cpus=0-2,cpus=5
+@end example
+
+@samp{mem} assigns a given RAM amount to a node. @samp{memdev}
+assigns RAM from a given memory backend device to a node. If
+@samp{mem} and @samp{memdev} are omitted in all nodes, RAM is
+split equally between them.
+
+@samp{mem} and @samp{memdev} are mutually exclusive. Furthermore,
+if one node uses @samp{memdev}, all of them have to use it.
+
+Note that the -@option{numa} option doesn't allocate any of the
+specified resources, it just assigns existing resources to NUMA
+nodes. This means that one still has to use the @option{-m},
+@option{-smp} options to allocate RAM and VCPUs respectively.
+
 ETEXI
 
 DEF("add-fd", HAS_ARG, QEMU_OPTION_add_fd,