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@@ -0,0 +1,123 @@
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+/*
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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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+
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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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+
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+#define TYPE_MEMORY_BACKEND_SHM "memory-backend-shm"
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+
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+OBJECT_DECLARE_SIMPLE_TYPE(HostMemoryBackendShm, MEMORY_BACKEND_SHM)
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+
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+struct HostMemoryBackendShm {
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+ HostMemoryBackend parent_obj;
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+};
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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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+
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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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+
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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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+ /*
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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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+ /*
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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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+
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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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+ /*
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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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+
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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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+
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+ ram_flags = RAM_SHARED;
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+ ram_flags |= backend->reserve ? 0 : RAM_NORESERVE;
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+
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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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+
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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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+
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+ MEMORY_BACKEND(m)->share = true;
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+}
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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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+
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+ bc->alloc = shm_backend_memory_alloc;
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+}
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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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+
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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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+
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+type_init(register_types);
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Stefano Garzarella