4e647fa085
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> Acked-by: Stefan Hajnoczi <stefanha@redhat.com> Acked-by: Markus Armbruster <armbru@redhat.com> (QAPI schema) Signed-off-by: Stefano Garzarella <sgarzare@redhat.com> Message-Id: <20240618100519.145853-1-sgarzare@redhat.com> Reviewed-by: Michael S. Tsirkin <mst@redhat.com> Signed-off-by: Michael S. Tsirkin <mst@redhat.com>
124 lines
3.4 KiB
C
124 lines
3.4 KiB
C
/*
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* QEMU host POSIX shared memory object backend
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*
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* Copyright (C) 2024 Red Hat Inc
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*
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* Authors:
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* Stefano Garzarella <sgarzare@redhat.com>
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*
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* This work is licensed under the terms of the GNU GPL, version 2 or later.
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* See the COPYING file in the top-level directory.
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*/
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#include "qemu/osdep.h"
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#include "sysemu/hostmem.h"
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#include "qapi/error.h"
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#define TYPE_MEMORY_BACKEND_SHM "memory-backend-shm"
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OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendShm, MEMORY_BACKEND_SHM)
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struct HostMemoryBackendShm {
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HostMemoryBackend parent_obj;
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};
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static bool
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shm_backend_memory_alloc(HostMemoryBackend *backend, Error **errp)
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{
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g_autoptr(GString) shm_name = g_string_new(NULL);
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g_autofree char *backend_name = NULL;
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uint32_t ram_flags;
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int fd, oflag;
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mode_t mode;
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if (!backend->size) {
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error_setg(errp, "can't create shm backend with size 0");
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return false;
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}
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if (!backend->share) {
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error_setg(errp, "can't create shm backend with `share=off`");
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return false;
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}
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/*
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* Let's use `mode = 0` because we don't want other processes to open our
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* memory unless we share the file descriptor with them.
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*/
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mode = 0;
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oflag = O_RDWR | O_CREAT | O_EXCL;
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backend_name = host_memory_backend_get_name(backend);
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/*
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* Some operating systems allow creating anonymous POSIX shared memory
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* objects (e.g. FreeBSD provides the SHM_ANON constant), but this is not
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* defined by POSIX, so let's create a unique name.
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*
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* From Linux's shm_open(3) man-page:
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* For portable use, a shared memory object should be identified
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* by a name of the form /somename;"
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*/
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g_string_printf(shm_name, "/qemu-" FMT_pid "-shm-%s", getpid(),
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backend_name);
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fd = shm_open(shm_name->str, oflag, mode);
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if (fd < 0) {
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error_setg_errno(errp, errno,
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"failed to create POSIX shared memory");
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return false;
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}
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/*
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* We have the file descriptor, so we no longer need to expose the
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* POSIX shared memory object. However it will remain allocated as long as
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* there are file descriptors pointing to it.
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*/
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shm_unlink(shm_name->str);
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if (ftruncate(fd, backend->size) == -1) {
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error_setg_errno(errp, errno,
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"failed to resize POSIX shared memory to %" PRIu64,
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backend->size);
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close(fd);
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return false;
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}
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ram_flags = RAM_SHARED;
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ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
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return memory_region_init_ram_from_fd(&backend->mr, OBJECT(backend),
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backend_name, backend->size,
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ram_flags, fd, 0, errp);
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}
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static void
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shm_backend_instance_init(Object *obj)
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{
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HostMemoryBackendShm *m = MEMORY_BACKEND_SHM(obj);
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MEMORY_BACKEND(m)->share = true;
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}
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static void
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shm_backend_class_init(ObjectClass *oc, void *data)
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{
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HostMemoryBackendClass *bc = MEMORY_BACKEND_CLASS(oc);
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bc->alloc = shm_backend_memory_alloc;
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}
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static const TypeInfo shm_backend_info = {
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.name = TYPE_MEMORY_BACKEND_SHM,
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.parent = TYPE_MEMORY_BACKEND,
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.instance_init = shm_backend_instance_init,
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.class_init = shm_backend_class_init,
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.instance_size = sizeof(HostMemoryBackendShm),
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};
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static void register_types(void)
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{
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type_register_static(&shm_backend_info);
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}
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type_init(register_types);
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