shm_open() creates and opens a new POSIX shared memory object.
A POSIX shared memory object allows creating memory backend with an
associated file descriptor that can be shared with external processes
(e.g. vhost-user).
The new `memory-backend-shm` can be used as an alternative when
`memory-backend-memfd` is not available (Linux only), since shm_open()
should be provided by any POSIX-compliant operating system.
This backend mimics memfd, allocating memory that is practically
anonymous. In theory shm_open() requires a name, but this is allocated
for a short time interval and shm_unlink() is called right after
shm_open(). After that, only fd is shared with external processes
(e.g., vhost-user) as if it were associated with anonymous memory.
In the future we may also allow the user to specify the name to be
passed to shm_open(), but for now we keep the backend simple, mimicking
anonymous memory such as memfd.
Acked-by: David Hildenbrand <david@redhat.com>
Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
---
v4
- fail if we find "share=off" in shm_backend_memory_alloc() [David]
v3
- enriched commit message and documentation to highlight that we
want to mimic memfd (David)
---
docs/system/devices/vhost-user.rst | 5 +-
qapi/qom.json | 17 ++++
backends/hostmem-shm.c | 123 +++++++++++++++++++++++++++++
backends/meson.build | 1 +
qemu-options.hx | 13 +++
5 files changed, 157 insertions(+), 2 deletions(-)
create mode 100644 backends/hostmem-shm.c
diff --git a/docs/system/devices/vhost-user.rst b/docs/system/devices/vhost-user.rst
index 9b2da106ce..35259d8ec7 100644
--- a/docs/system/devices/vhost-user.rst
+++ b/docs/system/devices/vhost-user.rst
@@ -98,8 +98,9 @@ Shared memory object
In order for the daemon to access the VirtIO queues to process the
requests it needs access to the guest's address space. This is
-achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
-objects. A reference to a file-descriptor which can access this object
+achieved via the ``memory-backend-file``, ``memory-backend-memfd``, or
+``memory-backend-shm`` objects.
+A reference to a file-descriptor which can access this object
will be passed via the socket as part of the protocol negotiation.
Currently the shared memory object needs to match the size of the main
diff --git a/qapi/qom.json b/qapi/qom.json
index 38dde6d785..52df052df8 100644
--- a/qapi/qom.json
+++ b/qapi/qom.json
@@ -721,6 +721,19 @@
'*hugetlbsize': 'size',
'*seal': 'bool' } }
+##
+# @MemoryBackendShmProperties:
+#
+# Properties for memory-backend-shm objects.
+#
+# The @share boolean option is true by default with shm.
+#
+# Since: 9.1
+##
+{ 'struct': 'MemoryBackendShmProperties',
+ 'base': 'MemoryBackendProperties',
+ 'data': { } }
+
##
# @MemoryBackendEpcProperties:
#
@@ -985,6 +998,8 @@
{ 'name': 'memory-backend-memfd',
'if': 'CONFIG_LINUX' },
'memory-backend-ram',
+ { 'name': 'memory-backend-shm',
+ 'if': 'CONFIG_POSIX' },
'pef-guest',
{ 'name': 'pr-manager-helper',
'if': 'CONFIG_LINUX' },
@@ -1056,6 +1071,8 @@
'memory-backend-memfd': { 'type': 'MemoryBackendMemfdProperties',
'if': 'CONFIG_LINUX' },
'memory-backend-ram': 'MemoryBackendProperties',
+ 'memory-backend-shm': { 'type': 'MemoryBackendShmProperties',
+ 'if': 'CONFIG_POSIX' },
'pr-manager-helper': { 'type': 'PrManagerHelperProperties',
'if': 'CONFIG_LINUX' },
'qtest': 'QtestProperties',
diff --git a/backends/hostmem-shm.c b/backends/hostmem-shm.c
new file mode 100644
index 0000000000..374edc3db8
--- /dev/null
+++ b/backends/hostmem-shm.c
@@ -0,0 +1,123 @@
+/*
+ * QEMU host POSIX shared memory object backend
+ *
+ * Copyright (C) 2024 Red Hat Inc
+ *
+ * Authors:
+ * Stefano Garzarella <sgarzare@redhat.com>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2 or later.
+ * See the COPYING file in the top-level directory.
+ */
+
+#include "qemu/osdep.h"
+#include "sysemu/hostmem.h"
+#include "qapi/error.h"
+
+#define TYPE_MEMORY_BACKEND_SHM "memory-backend-shm"
+
+OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendShm, MEMORY_BACKEND_SHM)
+
+struct HostMemoryBackendShm {
+ HostMemoryBackend parent_obj;
+};
+
+static bool
+shm_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
+{
+ g_autoptr(GString) shm_name = g_string_new(NULL);
+ g_autofree char *backend_name = NULL;
+ uint32_t ram_flags;
+ int fd, oflag;
+ mode_t mode;
+
+ if (!backend->size) {
+ error_setg(errp, "can't create shm backend with size 0");
+ return false;
+ }
+
+ if (!backend->share) {
+ error_setg(errp, "can't create shm backend with `share=off`");
+ return false;
+ }
+
+ /*
+ * Let's use `mode = 0` because we don't want other processes to open our
+ * memory unless we share the file descriptor with them.
+ */
+ mode = 0;
+ oflag = O_RDWR | O_CREAT | O_EXCL;
+ backend_name = host_memory_backend_get_name(backend);
+
+ /*
+ * Some operating systems allow creating anonymous POSIX shared memory
+ * objects (e.g. FreeBSD provides the SHM_ANON constant), but this is not
+ * defined by POSIX, so let's create a unique name.
+ *
+ * From Linux's shm_open(3) man-page:
+ * For portable use, a shared memory object should be identified
+ * by a name of the form /somename;"
+ */
+ g_string_printf(shm_name, "/qemu-" FMT_pid "-shm-%s", getpid(),
+ backend_name);
+
+ fd = shm_open(shm_name->str, oflag, mode);
+ if (fd < 0) {
+ error_setg_errno(errp, errno,
+ "failed to create POSIX shared memory");
+ return false;
+ }
+
+ /*
+ * We have the file descriptor, so we no longer need to expose the
+ * POSIX shared memory object. However it will remain allocated as long as
+ * there are file descriptors pointing to it.
+ */
+ shm_unlink(shm_name->str);
+
+ if (ftruncate(fd, backend->size) == -1) {
+ error_setg_errno(errp, errno,
+ "failed to resize POSIX shared memory to %" PRIu64,
+ backend->size);
+ close(fd);
+ return false;
+ }
+
+ ram_flags = RAM_SHARED;
+ ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
+
+ return memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
+ backend_name, backend->size,
+ ram_flags, fd, 0, errp);
+}
+
+static void
+shm_backend_instance_init(Object *obj)
+{
+ HostMemoryBackendShm *m = MEMORY_BACKEND_SHM(obj);
+
+ MEMORY_BACKEND(m)->share = true;
+}
+
+static void
+shm_backend_class_init(ObjectClass *oc, void *data)
+{
+ HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
+
+ bc->alloc = shm_backend_memory_alloc;
+}
+
+static const TypeInfo shm_backend_info = {
+ .name = TYPE_MEMORY_BACKEND_SHM,
+ .parent = TYPE_MEMORY_BACKEND,
+ .instance_init = shm_backend_instance_init,
+ .class_init = shm_backend_class_init,
+ .instance_size = sizeof(HostMemoryBackendShm),
+};
+
+static void register_types(void)
+{
+ type_register_static(&shm_backend_info);
+}
+
+type_init(register_types);
diff --git a/backends/meson.build b/backends/meson.build
index 8b2b111497..3867b0d363 100644
--- a/backends/meson.build
+++ b/backends/meson.build
@@ -13,6 +13,7 @@ system_ss.add([files(
if host_os != 'windows'
system_ss.add(files('rng-random.c'))
system_ss.add(files('hostmem-file.c'))
+ system_ss.add([files('hostmem-shm.c'), rt])
endif
if host_os == 'linux'
system_ss.add(files('hostmem-memfd.c'))
diff --git a/qemu-options.hx b/qemu-options.hx
index cf61f6b863..2226172fb0 100644
--- a/qemu-options.hx
+++ b/qemu-options.hx
@@ -5227,6 +5227,19 @@ SRST
The ``share`` boolean option is on by default with memfd.
+ ``-object memory-backend-shm,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 POSIX shared memory backend object, which allows
+ QEMU to share the memory with an external process (e.g. when
+ using vhost-user). This backend mimics memfd, allocating memory that is
+ practically anonymous. This is useful when memfd is not available.
+
+ Please refer to ``memory-backend-file`` for a description of the
+ options.
+
+ The ``share`` boolean option is on by default with shm. Setting it to
+ off will cause a failure during allocation because it is not supported
+ by this backend.
+
``-object iommufd,id=id[,fd=fd]``
Creates an iommufd backend which allows control of DMA mapping
through the ``/dev/iommu`` device.
--
2.45.0Stefano Garzarella <sgarzare@redhat.com> writes:
> shm_open() creates and opens a new POSIX shared memory object.
> A POSIX shared memory object allows creating memory backend with an
> associated file descriptor that can be shared with external processes
> (e.g. vhost-user).
>
> The new `memory-backend-shm` can be used as an alternative when
> `memory-backend-memfd` is not available (Linux only), since shm_open()
> should be provided by any POSIX-compliant operating system.
>
> This backend mimics memfd, allocating memory that is practically
> anonymous. In theory shm_open() requires a name, but this is allocated
> for a short time interval and shm_unlink() is called right after
> shm_open(). After that, only fd is shared with external processes
> (e.g., vhost-user) as if it were associated with anonymous memory.
>
> In the future we may also allow the user to specify the name to be
> passed to shm_open(), but for now we keep the backend simple, mimicking
> anonymous memory such as memfd.
>
> Acked-by: David Hildenbrand <david@redhat.com>
> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> ---
> v4
> - fail if we find "share=off" in shm_backend_memory_alloc() [David]
> v3
> - enriched commit message and documentation to highlight that we
> want to mimic memfd (David)
> ---
> docs/system/devices/vhost-user.rst | 5 +-
> qapi/qom.json | 17 ++++
> backends/hostmem-shm.c | 123 +++++++++++++++++++++++++++++
> backends/meson.build | 1 +
> qemu-options.hx | 13 +++
> 5 files changed, 157 insertions(+), 2 deletions(-)
> create mode 100644 backends/hostmem-shm.c
>
> diff --git a/docs/system/devices/vhost-user.rst b/docs/system/devices/vhost-user.rst
> index 9b2da106ce..35259d8ec7 100644
> --- a/docs/system/devices/vhost-user.rst
> +++ b/docs/system/devices/vhost-user.rst
> @@ -98,8 +98,9 @@ Shared memory object
>
> In order for the daemon to access the VirtIO queues to process the
> requests it needs access to the guest's address space. This is
> -achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
> -objects. A reference to a file-descriptor which can access this object
> +achieved via the ``memory-backend-file``, ``memory-backend-memfd``, or
> +``memory-backend-shm`` objects.
> +A reference to a file-descriptor which can access this object
> will be passed via the socket as part of the protocol negotiation.
>
> Currently the shared memory object needs to match the size of the main
> diff --git a/qapi/qom.json b/qapi/qom.json
> index 38dde6d785..52df052df8 100644
> --- a/qapi/qom.json
> +++ b/qapi/qom.json
> @@ -721,6 +721,19 @@
> '*hugetlbsize': 'size',
> '*seal': 'bool' } }
>
> +##
> +# @MemoryBackendShmProperties:
> +#
> +# Properties for memory-backend-shm objects.
> +#
> +# The @share boolean option is true by default with shm.
This contradicts the doc comment for @share:
# @share: if false, the memory is private to QEMU; if true, it is
# shared (default: false)
Your intention is to override that text. But that's less than clear.
Moreover, the documentation of @share is pretty far from this override.
John Snow is working on patches that'll pull it closer.
Hmm, MemoryBackendMemfdProperties has the same override.
I think we should change the doc comment for @share to something like
# @share: if false, the memory is private to QEMU; if true, it is
# shared (default depends on the backend type)
and then document the actual default with each backend type.
> +#
> +# Since: 9.1
> +##
> +{ 'struct': 'MemoryBackendShmProperties',
> + 'base': 'MemoryBackendProperties',
> + 'data': { } }
Let's add 'if': 'CONFIG_POSIX' here.
> +
> ##
> # @MemoryBackendEpcProperties:
> #
> @@ -985,6 +998,8 @@
> { 'name': 'memory-backend-memfd',
> 'if': 'CONFIG_LINUX' },
> 'memory-backend-ram',
> + { 'name': 'memory-backend-shm',
> + 'if': 'CONFIG_POSIX' },
> 'pef-guest',
> { 'name': 'pr-manager-helper',
> 'if': 'CONFIG_LINUX' },
> @@ -1056,6 +1071,8 @@
> 'memory-backend-memfd': { 'type': 'MemoryBackendMemfdProperties',
> 'if': 'CONFIG_LINUX' },
> 'memory-backend-ram': 'MemoryBackendProperties',
> + 'memory-backend-shm': { 'type': 'MemoryBackendShmProperties',
> + 'if': 'CONFIG_POSIX' },
> 'pr-manager-helper': { 'type': 'PrManagerHelperProperties',
> 'if': 'CONFIG_LINUX' },
> 'qtest': 'QtestProperties',
[...]
> diff --git a/qemu-options.hx b/qemu-options.hx
> index cf61f6b863..2226172fb0 100644
> --- a/qemu-options.hx
> +++ b/qemu-options.hx
> @@ -5227,6 +5227,19 @@ SRST
>
> The ``share`` boolean option is on by default with memfd.
>
> + ``-object memory-backend-shm,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 POSIX shared memory backend object, which allows
> + QEMU to share the memory with an external process (e.g. when
> + using vhost-user). This backend mimics memfd, allocating memory that is
> + practically anonymous. This is useful when memfd is not available.
This actually explains the purpose, unlike the doc comment in qom.json.
Same for the existing memory backends; can't fault you for doing your
new one the same way. We ought to fix them all. I'm not demanding you
do it.
The text could perhaps a bit clearer. What does "practically anonymous"
mean? As far as I understand, memory-backend-shm is a more portable and
less featureful version of memory-backend-memfd. Say that?
> +
> + Please refer to ``memory-backend-file`` for a description of the
> + options.
> +
> + The ``share`` boolean option is on by default with shm. Setting it to
> + off will cause a failure during allocation because it is not supported
> + by this backend.
> +
The "will case a failure" part is documented only here, and not in
qom.json.
Not this patch's fault: documentation for -object memory-backend-epc is
missing.
> ``-object iommufd,id=id[,fd=fd]``
> Creates an iommufd backend which allows control of DMA mapping
> through the ``/dev/iommu`` device.
On Wed, May 08, 2024 at 01:59:33PM GMT, Markus Armbruster wrote:
>Stefano Garzarella <sgarzare@redhat.com> writes:
>
>> shm_open() creates and opens a new POSIX shared memory object.
>> A POSIX shared memory object allows creating memory backend with an
>> associated file descriptor that can be shared with external processes
>> (e.g. vhost-user).
>>
>> The new `memory-backend-shm` can be used as an alternative when
>> `memory-backend-memfd` is not available (Linux only), since shm_open()
>> should be provided by any POSIX-compliant operating system.
>>
>> This backend mimics memfd, allocating memory that is practically
>> anonymous. In theory shm_open() requires a name, but this is allocated
>> for a short time interval and shm_unlink() is called right after
>> shm_open(). After that, only fd is shared with external processes
>> (e.g., vhost-user) as if it were associated with anonymous memory.
>>
>> In the future we may also allow the user to specify the name to be
>> passed to shm_open(), but for now we keep the backend simple, mimicking
>> anonymous memory such as memfd.
>>
>> Acked-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
>> ---
>> v4
>> - fail if we find "share=off" in shm_backend_memory_alloc() [David]
>> v3
>> - enriched commit message and documentation to highlight that we
>> want to mimic memfd (David)
>> ---
>> docs/system/devices/vhost-user.rst | 5 +-
>> qapi/qom.json | 17 ++++
>> backends/hostmem-shm.c | 123 +++++++++++++++++++++++++++++
>> backends/meson.build | 1 +
>> qemu-options.hx | 13 +++
>> 5 files changed, 157 insertions(+), 2 deletions(-)
>> create mode 100644 backends/hostmem-shm.c
>>
>> diff --git a/docs/system/devices/vhost-user.rst b/docs/system/devices/vhost-user.rst
>> index 9b2da106ce..35259d8ec7 100644
>> --- a/docs/system/devices/vhost-user.rst
>> +++ b/docs/system/devices/vhost-user.rst
>> @@ -98,8 +98,9 @@ Shared memory object
>>
>> In order for the daemon to access the VirtIO queues to process the
>> requests it needs access to the guest's address space. This is
>> -achieved via the ``memory-backend-file`` or ``memory-backend-memfd``
>> -objects. A reference to a file-descriptor which can access this object
>> +achieved via the ``memory-backend-file``, ``memory-backend-memfd``, or
>> +``memory-backend-shm`` objects.
>> +A reference to a file-descriptor which can access this object
>> will be passed via the socket as part of the protocol negotiation.
>>
>> Currently the shared memory object needs to match the size of the main
>> diff --git a/qapi/qom.json b/qapi/qom.json
>> index 38dde6d785..52df052df8 100644
>> --- a/qapi/qom.json
>> +++ b/qapi/qom.json
>> @@ -721,6 +721,19 @@
>> '*hugetlbsize': 'size',
>> '*seal': 'bool' } }
>>
>> +##
>> +# @MemoryBackendShmProperties:
>> +#
>> +# Properties for memory-backend-shm objects.
>> +#
>> +# The @share boolean option is true by default with shm.
>
>This contradicts the doc comment for @share:
>
> # @share: if false, the memory is private to QEMU; if true, it is
> # shared (default: false)
>
>Your intention is to override that text. But that's less than clear.
>Moreover, the documentation of @share is pretty far from this override.
>John Snow is working on patches that'll pull it closer.
>
>Hmm, MemoryBackendMemfdProperties has the same override.
Yep, I followed @MemoryBackendMemfdProperties and
@MemoryBackendEpcProperties.
>
>I think we should change the doc comment for @share to something like
>
> # @share: if false, the memory is private to QEMU; if true, it is
> # shared (default depends on the backend type)
>
>and then document the actual default with each backend type.
I agree on that, but I think we should do in a separate series/patch.
If you prefer, I can send that patch.
>
>> +#
>> +# Since: 9.1
>> +##
>> +{ 'struct': 'MemoryBackendShmProperties',
>> + 'base': 'MemoryBackendProperties',
>> + 'data': { } }
>
>Let's add 'if': 'CONFIG_POSIX' here.
>
Do you mean something like this:
{ 'struct': 'MemoryBackendShmProperties',
'if': 'CONFIG_POSIX',
'base': 'MemoryBackendProperties',
'data': { } }
I didn't because for MemoryBackendMemfdProperties and
MemoryBackendEpcProperties we have 'if': 'CONFIG_POSIX' only later in
the ObjectOptions union, so I did the same.
Should we fix them as well?
>> +
>> ##
>> # @MemoryBackendEpcProperties:
>> #
>> @@ -985,6 +998,8 @@
>> { 'name': 'memory-backend-memfd',
>> 'if': 'CONFIG_LINUX' },
>> 'memory-backend-ram',
>> + { 'name': 'memory-backend-shm',
>> + 'if': 'CONFIG_POSIX' },
>> 'pef-guest',
>> { 'name': 'pr-manager-helper',
>> 'if': 'CONFIG_LINUX' },
>> @@ -1056,6 +1071,8 @@
>> 'memory-backend-memfd': { 'type': 'MemoryBackendMemfdProperties',
>> 'if': 'CONFIG_LINUX' },
>> 'memory-backend-ram': 'MemoryBackendProperties',
>> + 'memory-backend-shm': { 'type': 'MemoryBackendShmProperties',
>> + 'if': 'CONFIG_POSIX' },
>> 'pr-manager-helper': { 'type': 'PrManagerHelperProperties',
>> 'if': 'CONFIG_LINUX' },
>> 'qtest': 'QtestProperties',
>
>[...]
>
>> diff --git a/qemu-options.hx b/qemu-options.hx
>> index cf61f6b863..2226172fb0 100644
>> --- a/qemu-options.hx
>> +++ b/qemu-options.hx
>> @@ -5227,6 +5227,19 @@ SRST
>>
>> The ``share`` boolean option is on by default with memfd.
>>
>> + ``-object memory-backend-shm,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 POSIX shared memory backend object, which allows
>> + QEMU to share the memory with an external process (e.g. when
>> + using vhost-user). This backend mimics memfd, allocating memory that is
>> + practically anonymous. This is useful when memfd is not available.
>
>This actually explains the purpose, unlike the doc comment in qom.json.
>Same for the existing memory backends; can't fault you for doing your
>new one the same way. We ought to fix them all. I'm not demanding you
>do it.
>
>The text could perhaps a bit clearer. What does "practically anonymous"
>mean? As far as I understand, memory-backend-shm is a more portable and
>less featureful version of memory-backend-memfd. Say that?
Yeah, I see, I'll fix it with your suggestion!
>
>> +
>> + Please refer to ``memory-backend-file`` for a description of the
>> + options.
>> +
>> + The ``share`` boolean option is on by default with shm. Setting it to
>> + off will cause a failure during allocation because it is not supported
>> + by this backend.
>> +
>
>The "will case a failure" part is documented only here, and not in
>qom.json.
I'll add it also in qom.json.
>
>Not this patch's fault: documentation for -object memory-backend-epc is
>missing.
Oh right, I don't know anything about it. Should we ping Sean
Christopherson <sean.j.christopherson@intel.com> who added it?
Thanks,
Stefano
>
>> ``-object iommufd,id=id[,fd=fd]``
>> Creates an iommufd backend which allows control of DMA mapping
>> through the ``/dev/iommu`` device.
>
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