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/*
* 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 "system/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);
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