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machine: add missing doc for memory-backend option

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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>
This commit is contained in:
Igor Mammedov 2021-01-21 11:15:04 -05:00 committed by Paolo Bonzini
parent 5ea9e9e239
commit 8db0b20415
2 changed files with 35 additions and 1 deletions
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10
backends/hostmem.c
View File

@ -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);

26
qemu-options.hx
View File

@ -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