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Tsirkin" , Igor Mammedov , =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , Stefano Garzarella Subject: [PATCH v4 10/12] hostmem: add a new memory backend based on POSIX shm_open() Date: Wed, 8 May 2024 09:44:54 +0200 Message-ID: <20240508074457.12367-11-sgarzare@redhat.com> X-Mailer: git-send-email 2.45.0 In-Reply-To: <20240508074457.12367-1-sgarzare@redhat.com> References: <20240508074457.12367-1-sgarzare@redhat.com> MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Received-SPF: pass (zohomail.com: domain of gnu.org designates 209.51.188.17 as permitted sender) client-ip=209.51.188.17; envelope-from=qemu-devel-bounces+importer=patchew.org@nongnu.org; helo=lists.gnu.org; Received-SPF: pass client-ip=170.10.129.124; envelope-from=sgarzare@redhat.com; helo=us-smtp-delivery-124.mimecast.com X-Spam_score_int: -26 X-Spam_score: -2.7 X-Spam_bar: -- X-Spam_report: (-2.7 / 5.0 requ) BAYES_00=-1.9, DKIMWL_WL_HIGH=-0.581, DKIM_SIGNED=0.1, DKIM_VALID=-0.1, DKIM_VALID_AU=-0.1, DKIM_VALID_EF=-0.1, RCVD_IN_DNSWL_NONE=-0.0001, RCVD_IN_MSPIKE_H4=0.001, RCVD_IN_MSPIKE_WL=0.001, SPF_HELO_NONE=0.001, SPF_PASS=-0.001 autolearn=ham autolearn_force=no X-Spam_action: no action X-BeenThere: qemu-devel@nongnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Errors-To: qemu-devel-bounces+importer=patchew.org@nongnu.org Sender: qemu-devel-bounces+importer=patchew.org@nongnu.org X-ZohoMail-DKIM: pass (identity @redhat.com) X-ZM-MESSAGEID: 1715154430207100001 Content-Type: text/plain; charset="utf-8" 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 Signed-off-by: Stefano Garzarella --- v4 - fail if we find "share=3Doff" 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 =20 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. =20 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' } } =20 +## +# @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': 'MemoryBackendMemfdPropertie= s', '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 + * + * This work is licensed under the terms of the GNU GPL, version 2 or late= r. + * 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 =3D g_string_new(NULL); + g_autofree char *backend_name =3D 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=3Doff`"); + return false; + } + + /* + * Let's use `mode =3D 0` because we don't want other processes to ope= n our + * memory unless we share the file descriptor with them. + */ + mode =3D 0; + oflag =3D O_RDWR | O_CREAT | O_EXCL; + backend_name =3D 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 =3D 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 lon= g as + * there are file descriptors pointing to it. + */ + shm_unlink(shm_name->str); + + if (ftruncate(fd, backend->size) =3D=3D -1) { + error_setg_errno(errp, errno, + "failed to resize POSIX shared memory to %" PRIu6= 4, + backend->size); + close(fd); + return false; + } + + ram_flags =3D RAM_SHARED; + ram_flags |=3D 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 =3D MEMORY_BACKEND_SHM(obj); + + MEMORY_BACKEND(m)->share =3D true; +} + +static void +shm_backend_class_init(ObjectClass *oc, void *data) +{ + HostMemoryBackendClass *bc =3D MEMORY_BACKEND_CLASS(oc); + + bc->alloc =3D shm_backend_memory_alloc; +} + +static const TypeInfo shm_backend_info =3D { + .name =3D TYPE_MEMORY_BACKEND_SHM, + .parent =3D TYPE_MEMORY_BACKEND, + .instance_init =3D shm_backend_instance_init, + .class_init =3D shm_backend_class_init, + .instance_size =3D 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 !=3D '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 =3D=3D '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 =20 The ``share`` boolean option is on by default with memfd. =20 + ``-object memory-backend-shm,id=3Did,merge=3Don|off,dump=3Don|off,shar= e=3Don|off,prealloc=3Don|off,size=3Dsize,host-nodes=3Dhost-nodes,policy=3Dd= efault|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 th= at 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 suppo= rted + by this backend. + ``-object iommufd,id=3Did[,fd=3Dfd]`` Creates an iommufd backend which allows control of DMA mapping through the ``/dev/iommu`` device. --=20 2.45.0