@@ -18,6 +18,8 @@
#include "sysemu/hostmem.h"
#include "qom/object_interfaces.h"
#include "qom/object.h"
+#include "qapi/visitor.h"
+#include "qapi/qapi-visit-common.h"
OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendFile, MEMORY_BACKEND_FILE)
@@ -31,6 +33,7 @@ struct HostMemoryBackendFile {
bool discard_data;
bool is_pmem;
bool readonly;
+ OnOffAuto rom;
};
static void
@@ -53,9 +56,33 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
return;
}
+ switch (fb->rom) {
+ case ON_OFF_AUTO_AUTO:
+ /* Traditionally, opening the file readonly always resulted in ROM. */
+ fb->rom = fb->readonly ? ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
+ break;
+ case ON_OFF_AUTO_ON:
+ if (!fb->readonly) {
+ error_setg(errp, "property 'rom' = 'on' is not supported with"
+ " 'readonly' = 'off'");
+ return;
+ }
+ break;
+ case ON_OFF_AUTO_OFF:
+ if (fb->readonly && backend->share) {
+ error_setg(errp, "property 'rom' = 'off' is incompatible with"
+ " 'readonly' = 'on' and 'share' = 'on'");
+ return;
+ }
+ break;
+ default:
+ assert(false);
+ }
+
name = host_memory_backend_get_name(backend);
ram_flags = backend->share ? RAM_SHARED : 0;
- ram_flags |= fb->readonly ? RAM_READONLY | RAM_READONLY_FD : 0;
+ ram_flags |= fb->readonly ? RAM_READONLY_FD : 0;
+ ram_flags |= fb->rom == ON_OFF_AUTO_ON ? RAM_READONLY : 0;
ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
ram_flags |= fb->is_pmem ? RAM_PMEM : 0;
ram_flags |= RAM_NAMED_FILE;
@@ -201,6 +228,32 @@ static void file_memory_backend_set_readonly(Object *obj, bool value,
fb->readonly = value;
}
+static void file_memory_backend_get_rom(Object *obj, Visitor *v,
+ const char *name, void *opaque,
+ Error **errp)
+{
+ HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
+ OnOffAuto rom = fb->rom;
+
+ visit_type_OnOffAuto(v, name, &rom, errp);
+}
+
+static void file_memory_backend_set_rom(Object *obj, Visitor *v,
+ const char *name, void *opaque,
+ Error **errp)
+{
+ HostMemoryBackend *backend = MEMORY_BACKEND(obj);
+ HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
+
+ if (host_memory_backend_mr_inited(backend)) {
+ error_setg(errp, "cannot change property '%s' of %s.", name,
+ object_get_typename(obj));
+ return;
+ }
+
+ visit_type_OnOffAuto(v, name, &fb->rom, errp);
+}
+
static void file_backend_unparent(Object *obj)
{
HostMemoryBackend *backend = MEMORY_BACKEND(obj);
@@ -243,6 +296,10 @@ file_backend_class_init(ObjectClass *oc, void *data)
object_class_property_add_bool(oc, "readonly",
file_memory_backend_get_readonly,
file_memory_backend_set_readonly);
+ object_class_property_add(oc, "rom", "OnOffAuto",
+ file_memory_backend_get_rom, file_memory_backend_set_rom, NULL, NULL);
+ object_class_property_set_description(oc, "rom",
+ "Whether to create Read Only Memory (ROM)");
}
static void file_backend_instance_finalize(Object *o)
@@ -668,6 +668,20 @@
# @readonly: if true, the backing file is opened read-only; if false,
# it is opened read-write. (default: false)
#
+# @rom: whether to create Read Only Memory (ROM) that cannot be modified
+# by the VM. Any write attempts to such ROM will be denied. Most
+# use cases want writable RAM instead of ROM. However, selected use
+# cases, like R/O NVDIMMs, can benefit from ROM. If set to 'on',
+# create ROM; if set to 'off', create writable RAM; if set to
+# 'auto', the value of the @readonly property is used. This
+# property is primarily helpful when we want to have proper RAM in
+# configurations that would traditionally create ROM before this
+# property was introduced: VM templating, where we want to open a
+# file readonly (@readonly set to true) and mark the memory to be
+# private for QEMU (@share set to false). For this use case, we need
+# writable RAM instead of ROM, and want to set this property to 'off'.
+# (default: auto, since 8.2)
+#
# Since: 2.1
##
{ 'struct': 'MemoryBackendFileProperties',
@@ -677,7 +691,8 @@
'*discard-data': 'bool',
'mem-path': 'str',
'*pmem': { 'type': 'bool', 'if': 'CONFIG_LIBPMEM' },
- '*readonly': 'bool' } }
+ '*readonly': 'bool',
+ '*rom': 'OnOffAuto' } }
##
# @MemoryBackendMemfdProperties:
@@ -4976,7 +4976,7 @@ SRST
they are specified. Note that the 'id' property must be set. These
objects are placed in the '/objects' path.
- ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,offset=offset,readonly=on|off``
+ ``-object memory-backend-file,id=id,size=size,mem-path=dir,share=on|off,discard-data=on|off,merge=on|off,dump=on|off,prealloc=on|off,host-nodes=host-nodes,policy=default|preferred|bind|interleave,align=align,offset=offset,readonly=on|off,rom=on|off|auto``
Creates a memory file backend object, which can be used to back
the guest RAM with huge pages.
@@ -5066,6 +5066,20 @@ SRST
The ``readonly`` option specifies whether the backing file is opened
read-only or read-write (default).
+ The ``rom`` option specifies whether to create Read Only Memory
+ (ROM) that cannot be modified by the VM. Any write attempts to such
+ ROM will be denied. Most use cases want proper RAM instead of ROM.
+ However, selected use cases, like R/O NVDIMMs, can benefit from
+ ROM. If set to ``on``, create ROM; if set to ``off``, create
+ writable RAM; if set to ``auto`` (default), the value of the
+ ``readonly`` option is used. This option is primarily helpful when
+ we want to have writable RAM in configurations that would
+ traditionally create ROM before the ``rom`` option was introduced:
+ VM templating, where we want to open a file readonly
+ (``readonly=on``) and mark the memory to be private for QEMU
+ (``share=off``). For this use case, we need writable RAM instead
+ of ROM, and want to also set ``rom=off``.
+
``-object memory-backend-ram,id=id,merge=on|off,dump=on|off,share=on|off,prealloc=on|off,size=size,host-nodes=host-nodes,policy=default|preferred|bind|interleave``
Creates a memory backend object, which can be used to back the
guest RAM. Memory backend objects offer more control than the
For now, "share=off,readonly=on" would always result in us opening the file R/O and mmap'ing the opened file MAP_PRIVATE R/O -- effectively turning it into ROM. Especially for VM templating, "share=off" is a common use case. However, that use case is impossible with files that lack write permissions, because "share=off,readonly=on" will not give us writable RAM. The sole user of ROM via memory-backend-file are R/O NVDIMMs, but as we have users (Kata Containers) that rely on the existing behavior -- malicious VMs should not be able to consume COW memory for R/O NVDIMMs -- we cannot change the semantics of "share=off,readonly=on" So let's add a new "rom" property with on/off/auto values. "auto" is the default and what most people will use: for historical reasons, to not change the old semantics, it defaults to the value of the "readonly" property. For VM templating, one can now use: -object memory-backend-file,share=off,readonly=on,rom=off,... But we'll disallow: -object memory-backend-file,share=on,readonly=on,rom=off,... because we would otherwise get an error when trying to mmap the R/O file shared and writable. An explicit error message is cleaner. We will also disallow for now: -object memory-backend-file,share=off,readonly=off,rom=on,... -object memory-backend-file,share=on,readonly=off,rom=on,... It's not harmful, but also not really required for now. Alternatives that were abandoned: * Make "unarmed=on" for the NVDIMM set the memory region container readonly. We would still see a change of ROM->RAM and possibly run into memslot limits with vhost-user. Further, there might be use cases for "unarmed=on" that should still allow writing to that memory (temporary files, system RAM, ...). * Add a new "readonly=on/off/auto" parameter for NVDIMMs. Similar issues as with "unarmed=on". * Make "readonly" consume "on/off/file" instead of being a 'bool' type. This would slightly changes the behavior of the "readonly" parameter: values like true/false (as accepted by a 'bool'type) would no longer be accepted. Acked-by: Markus Armbruster <armbru@redhat.com> Signed-off-by: David Hildenbrand <david@redhat.com> --- backends/hostmem-file.c | 59 ++++++++++++++++++++++++++++++++++++++++- qapi/qom.json | 17 +++++++++++- qemu-options.hx | 16 ++++++++++- 3 files changed, 89 insertions(+), 3 deletions(-)