backends/hostmem-file: Add "rom" property to support VM templating with 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>
This commit is contained in:
David Hildenbrand
parent
5c52a219bb
commit
e92666b0ba
@ -18,6 +18,8 @@
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#include "sysemu/hostmem.h"
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#include "qom/object_interfaces.h"
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#include "qom/object.h"
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#include "qapi/visitor.h"
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#include "qapi/qapi-visit-common.h"
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OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendFile, MEMORY_BACKEND_FILE)
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@ -31,6 +33,7 @@ struct HostMemoryBackendFile {
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bool discard_data;
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bool is_pmem;
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bool readonly;
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OnOffAuto rom;
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};
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static void
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@ -53,9 +56,33 @@ file_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
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return;
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}
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switch (fb->rom) {
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case ON_OFF_AUTO_AUTO:
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/* Traditionally, opening the file readonly always resulted in ROM. */
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fb->rom = fb->readonly ? ON_OFF_AUTO_ON : ON_OFF_AUTO_OFF;
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break;
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case ON_OFF_AUTO_ON:
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if (!fb->readonly) {
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error_setg(errp, "property 'rom' = 'on' is not supported with"
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" 'readonly' = 'off'");
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return;
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}
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break;
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case ON_OFF_AUTO_OFF:
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if (fb->readonly && backend->share) {
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error_setg(errp, "property 'rom' = 'off' is incompatible with"
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" 'readonly' = 'on' and 'share' = 'on'");
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return;
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}
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break;
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default:
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assert(false);
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}
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name = host_memory_backend_get_name(backend);
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ram_flags = backend->share ? RAM_SHARED : 0;
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ram_flags |= fb->readonly ? RAM_READONLY | RAM_READONLY_FD : 0;
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ram_flags |= fb->readonly ? RAM_READONLY_FD : 0;
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ram_flags |= fb->rom == ON_OFF_AUTO_ON ? RAM_READONLY : 0;
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ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
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ram_flags |= fb->is_pmem ? RAM_PMEM : 0;
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ram_flags |= RAM_NAMED_FILE;
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@ -201,6 +228,32 @@ static void file_memory_backend_set_readonly(Object *obj, bool value,
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fb->readonly = value;
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}
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static void file_memory_backend_get_rom(Object *obj, Visitor *v,
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const char *name, void *opaque,
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Error **errp)
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{
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HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
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OnOffAuto rom = fb->rom;
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visit_type_OnOffAuto(v, name, &rom, errp);
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}
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static void file_memory_backend_set_rom(Object *obj, Visitor *v,
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const char *name, void *opaque,
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Error **errp)
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{
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HostMemoryBackend *backend = MEMORY_BACKEND(obj);
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HostMemoryBackendFile *fb = MEMORY_BACKEND_FILE(obj);
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if (host_memory_backend_mr_inited(backend)) {
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error_setg(errp, "cannot change property '%s' of %s.", name,
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object_get_typename(obj));
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return;
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}
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visit_type_OnOffAuto(v, name, &fb->rom, errp);
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}
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static void file_backend_unparent(Object *obj)
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{
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HostMemoryBackend *backend = MEMORY_BACKEND(obj);
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@ -243,6 +296,10 @@ file_backend_class_init(ObjectClass *oc, void *data)
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object_class_property_add_bool(oc, "readonly",
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file_memory_backend_get_readonly,
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file_memory_backend_set_readonly);
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object_class_property_add(oc, "rom", "OnOffAuto",
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file_memory_backend_get_rom, file_memory_backend_set_rom, NULL, NULL);
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object_class_property_set_description(oc, "rom",
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"Whether to create Read Only Memory (ROM)");
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}
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static void file_backend_instance_finalize(Object *o)
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@ -668,6 +668,20 @@
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# @readonly: if true, the backing file is opened read-only; if false,
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# it is opened read-write. (default: false)
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#
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# @rom: whether to create Read Only Memory (ROM) that cannot be modified
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# by the VM. Any write attempts to such ROM will be denied. Most
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# use cases want writable RAM instead of ROM. However, selected use
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# cases, like R/O NVDIMMs, can benefit from ROM. If set to 'on',
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# create ROM; if set to 'off', create writable RAM; if set to
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# 'auto', the value of the @readonly property is used. This
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# property is primarily helpful when we want to have proper RAM in
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# configurations that would traditionally create ROM before this
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# property was introduced: VM templating, where we want to open a
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# file readonly (@readonly set to true) and mark the memory to be
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# private for QEMU (@share set to false). For this use case, we need
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# writable RAM instead of ROM, and want to set this property to 'off'.
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# (default: auto, since 8.2)
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#
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# Since: 2.1
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##
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{ 'struct': 'MemoryBackendFileProperties',
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@ -677,7 +691,8 @@
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'*discard-data': 'bool',
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'mem-path': 'str',
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'*pmem': { 'type': 'bool', 'if': 'CONFIG_LIBPMEM' },
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'*readonly': 'bool' } }
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'*readonly': 'bool',
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'*rom': 'OnOffAuto' } }
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##
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# @MemoryBackendMemfdProperties:
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@ -4995,7 +4995,7 @@ SRST
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they are specified. Note that the 'id' property must be set. These
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objects are placed in the '/objects' path.
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``-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``
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``-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``
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Creates a memory file backend object, which can be used to back
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the guest RAM with huge pages.
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@ -5085,6 +5085,20 @@ SRST
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The ``readonly`` option specifies whether the backing file is opened
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read-only or read-write (default).
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The ``rom`` option specifies whether to create Read Only Memory
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(ROM) that cannot be modified by the VM. Any write attempts to such
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ROM will be denied. Most use cases want proper RAM instead of ROM.
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However, selected use cases, like R/O NVDIMMs, can benefit from
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ROM. If set to ``on``, create ROM; if set to ``off``, create
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writable RAM; if set to ``auto`` (default), the value of the
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``readonly`` option is used. This option is primarily helpful when
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we want to have writable RAM in configurations that would
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traditionally create ROM before the ``rom`` option was introduced:
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VM templating, where we want to open a file readonly
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(``readonly=on``) and mark the memory to be private for QEMU
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(``share=off``). For this use case, we need writable RAM instead
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of ROM, and want to also set ``rom=off``.
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``-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``
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Creates a memory backend object, which can be used to back the
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guest RAM. Memory backend objects offer more control than the
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