machine: add missing doc for memory-backend option

Add documentation for '-machine memory-backend' CLI option and
how to use it.

And document that x-use-canonical-path-for-ramblock-id,
is considered to be stable to make sure it won't go away by accident.

x- was intended for unstable/iternal properties, and not supposed to
be stable option. However it's too late to rename (drop x-)
it as it would mean that users will have to mantain both
x-use-canonical-path-for-ramblock-id (for QEMU 5.0-5.2) versions
and prefix-less for later versions.

Signed-off-by: Igor Mammedov <imammedo@redhat.com>
Message-Id: <20210121161504.1007247-1-imammedo@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

backends/hostmem.c

@@ -494,6 +494,16 @@ host_memory_backend_class_init(ObjectClass *oc, void *data)
         host_memory_backend_get_share, host_memory_backend_set_share);
     object_class_property_set_description(oc, "share",
         "Mark the memory as private to QEMU or shared");
+    /*
+     * Do not delete/rename option. This option must be considered stable
+     * (as if it didn't have the 'x-' prefix including deprecation period) as
+     * long as 4.0 and older machine types exists.
+     * Option will be used by upper layers to override (disable) canonical path
+     * for ramblock-id set by compat properties on old machine types ( <= 4.0),
+     * to keep migration working when backend is used for main RAM with
+     * -machine memory-backend= option (main RAM historically used prefix-less
+     * ramblock-id).
+     */
     object_class_property_add_bool(oc, "x-use-canonical-path-for-ramblock-id",
         host_memory_backend_get_use_canonical_path,
         host_memory_backend_set_use_canonical_path);

qemu-options.hx

@@ -35,7 +35,8 @@ DEF("machine", HAS_ARG, QEMU_OPTION_machine, \
     "                suppress-vmdesc=on|off disables self-describing migration (default=off)\n"
     "                nvdimm=on|off controls NVDIMM support (default=off)\n"
     "                memory-encryption=@var{} memory encryption object to use (default=none)\n"
-    "                hmat=on|off controls ACPI HMAT support (default=off)\n",
+    "                hmat=on|off controls ACPI HMAT support (default=off)\n"
+    "                memory-backend='backend-id' specifies explicitly provided backend for main RAM (default=none)\n",
     QEMU_ARCH_ALL)
 SRST
 ``-machine [type=]name[,prop=value[,...]]``
@@ -96,6 +97,29 @@ SRST
     ``hmat=on|off``
         Enables or disables ACPI Heterogeneous Memory Attribute Table
         (HMAT) support. The default is off.
+
+     ``memory-backend='id'``
+        An alternative to legacy ``-mem-path`` and ``mem-prealloc`` options.
+        Allows to use a memory backend as main RAM.
+
+        For example:
+        ::
+        -object memory-backend-file,id=pc.ram,size=512M,mem-path=/hugetlbfs,prealloc=on,share=on
+        -machine memory-backend=pc.ram
+        -m 512M
+
+        Migration compatibility note:
+        a) as backend id one shall use value of 'default-ram-id', advertised by
+        machine type (available via ``query-machines`` QMP command), if migration
+        to/from old QEMU (<5.0) is expected.
+        b) for machine types 4.0 and older, user shall
+        use ``x-use-canonical-path-for-ramblock-id=off`` backend option
+        if migration to/from old QEMU (<5.0) is expected.
+        For example:
+        ::
+        -object memory-backend-ram,id=pc.ram,size=512M,x-use-canonical-path-for-ramblock-id=off
+        -machine memory-backend=pc.ram
+        -m 512M
 ERST
 
 HXCOMM Deprecated by -machine