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Tue, 18 Jun 2024 03:05:24 -0700 (PDT) X-Google-Smtp-Source: AGHT+IGMYlPQiOMCFpfcpAKB7tB+M7b66KpVX5mWAo18HsFUCXejozZz1ohpgI74z8orq/HoQHt6nQ== X-Received: by 2002:a05:600c:1553:b0:422:1609:a7db with SMTP id 5b1f17b1804b1-42304822a02mr94547995e9.8.1718705124594; Tue, 18 Jun 2024 03:05:24 -0700 (PDT) From: Stefano Garzarella To: mst@redhat.com, qemu-devel@nongnu.org Cc: Thomas Huth , Kevin Wolf , qemu-block@nongnu.org, =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= , =?UTF-8?q?Daniel=20P=2E=20Berrang=C3=A9?= , Jason Wang , Gerd Hoffmann , =?UTF-8?q?Philippe=20Mathieu-Daud=C3=A9?= , Hanna Reitz , Brad Smith , David Hildenbrand , Paolo Bonzini , Eduardo Habkost , Laurent Vivier , Coiby Xu , slp@redhat.com, gmaglione@redhat.com, Igor Mammedov , stefanha@redhat.com, Raphael Norwitz , Markus Armbruster , Eric Blake , Stefano Garzarella Subject: [PATCH v8 11/13] hostmem: add a new memory backend based on POSIX shm_open() Date: Tue, 18 Jun 2024 12:05:19 +0200 Message-ID: <20240618100519.145853-1-sgarzare@redhat.com> X-Mailer: git-send-email 2.45.2 In-Reply-To: <20240618100043.144657-1-sgarzare@redhat.com> References: <20240618100043.144657-1-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; 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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 Acked-by: Stefan Hajnoczi Acked-by: Markus Armbruster (QAPI schema) Signed-off-by: Stefano Garzarella --- v8 - Fixed QAPI documentation about share option [Markus] v7 - changed default value documentation for @share rebasing on 20240611130231.83152-1-sgarzare@redhat.com [Markus] - used `memory-backend-shm` instead of `shm` and wrapped to 70 columns [Markus] - added 'if': 'CONFIG_POSIX' to MemoryBackendShmProperties [Markus] v5 - fixed documentation in qapi/qom.json and qemu-options.hx [Markus] 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 | 23 +++++- backends/hostmem-shm.c | 123 +++++++++++++++++++++++++++++ backends/meson.build | 1 + qemu-options.hx | 16 ++++ 5 files changed, 164 insertions(+), 4 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 9b8f6a7ab5..92b0fea76c 100644 --- a/qapi/qom.json +++ b/qapi/qom.json @@ -601,8 +601,8 @@ # # @share: if false, the memory is private to QEMU; if true, it is # shared (default false for backends memory-backend-file and -# memory-backend-ram, true for backends memory-backend-epc and -# memory-backend-memfd) +# memory-backend-ram, true for backends memory-backend-epc, +# memory-backend-memfd, and memory-backend-shm) # # @reserve: if true, reserve swap space (or huge pages) if applicable # (default: true) (since 6.1) @@ -721,6 +721,21 @@ '*hugetlbsize': 'size', '*seal': 'bool' } } =20 +## +# @MemoryBackendShmProperties: +# +# Properties for memory-backend-shm objects. +# +# This memory backend supports only shared memory, which is the +# default. +# +# Since: 9.1 +## +{ 'struct': 'MemoryBackendShmProperties', + 'base': 'MemoryBackendProperties', + 'data': { }, + 'if': 'CONFIG_POSIX' } + ## # @MemoryBackendEpcProperties: # @@ -1049,6 +1064,8 @@ { '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' }, @@ -1121,6 +1138,8 @@ '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 8ca7f34ef0..ad6521ef5e 100644 --- a/qemu-options.hx +++ b/qemu-options.hx @@ -5240,6 +5240,22 @@ 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). + + ``memory-backend-shm`` is a more portable and less featureful vers= ion + of ``memory-backend-memfd``. It can then be used in any POSIX syst= em, + especially when memfd is not supported. + + 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 2.45.2