rng-builtin: add an RNG backend that uses qemu_guest_getrandom()

Add a new RNG backend using QEMU builtin getrandom function.

It can be created and used with something like:

    ... -object rng-builtin,id=rng0 -device virtio-rng,rng=rng0 ...
Signed-off-by: NLaurent Vivier <lvivier@redhat.com>
Message-Id: <20190820160615.14616-2-lvivier@redhat.com>

backends/Makefile.objs

-common-obj-y += rng.o rng-egd.o
+common-obj-y += rng.o rng-egd.o rng-builtin.o
 common-obj-$(CONFIG_POSIX) += rng-random.o
 
 common-obj-$(CONFIG_TPM) += tpm.o

backends/rng-builtin.c

+/*
+ * QEMU Builtin Random Number Generator Backend
+ *
+ * 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 "sysemu/rng.h"
+#include "qemu/main-loop.h"
+#include "qemu/guest-random.h"
+
+#define TYPE_RNG_BUILTIN "rng-builtin"
+#define RNG_BUILTIN(obj) OBJECT_CHECK(RngBuiltin, (obj), TYPE_RNG_BUILTIN)
+
+typedef struct RngBuiltin {
+    RngBackend parent;
+    QEMUBH *bh;
+} RngBuiltin;
+
+static void rng_builtin_receive_entropy_bh(void *opaque)
+{
+    RngBuiltin *s = opaque;
+
+    while (!QSIMPLEQ_EMPTY(&s->parent.requests)) {
+        RngRequest *req = QSIMPLEQ_FIRST(&s->parent.requests);
+
+        qemu_guest_getrandom_nofail(req->data, req->size);
+
+        req->receive_entropy(req->opaque, req->data, req->size);
+
+        rng_backend_finalize_request(&s->parent, req);
+    }
+}
+
+static void rng_builtin_request_entropy(RngBackend *b, RngRequest *req)
+{
+    RngBuiltin *s = RNG_BUILTIN(b);
+
+    qemu_bh_schedule(s->bh);
+}
+
+static void rng_builtin_init(Object *obj)
+{
+    RngBuiltin *s = RNG_BUILTIN(obj);
+
+    s->bh = qemu_bh_new(rng_builtin_receive_entropy_bh, s);
+}
+
+static void rng_builtin_finalize(Object *obj)
+{
+    RngBuiltin *s = RNG_BUILTIN(obj);
+
+    qemu_bh_delete(s->bh);
+}
+
+static void rng_builtin_class_init(ObjectClass *klass, void *data)
+{
+    RngBackendClass *rbc = RNG_BACKEND_CLASS(klass);
+
+    rbc->request_entropy = rng_builtin_request_entropy;
+}
+
+static const TypeInfo rng_builtin_info = {
+    .name = TYPE_RNG_BUILTIN,
+    .parent = TYPE_RNG_BACKEND,
+    .instance_size = sizeof(RngBuiltin),
+    .instance_init = rng_builtin_init,
+    .instance_finalize = rng_builtin_finalize,
+    .class_init = rng_builtin_class_init,
+};
+
+static void register_types(void)
+{
+    type_register_static(&rng_builtin_info);
+}
+
+type_init(register_types);

qemu-options.hx

@@ -4332,6 +4332,13 @@ other options.
 
 The @option{share} boolean option is @var{on} by default with memfd.
 
+@item -object rng-builtin,id=@var{id}
+
+Creates a random number generator backend which obtains entropy from
+QEMU builtin functions. The @option{id} parameter is a unique ID that
+will be used to reference this entropy backend from the @option{virtio-rng}
+device.
+
 @item -object rng-random,id=@var{id},filename=@var{/dev/random}
 
 Creates a random number generator backend which obtains entropy from