backends/hostmem-file: Add "rom" property to support VM templating wi…

…th R/O files

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.

Message-ID: <20230906120503.359863-4-david@redhat.com>
Acked-by: Markus Armbruster <armbru@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

backends/hostmem-file.c

@@ -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)

qapi/qom.json

@@ -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:

qemu-options.hx

@@ -4995,7 +4995,7 @@ 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.
 
@@ -5085,6 +5085,20 @@ 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