virtio_random: Pipeline fetching the data

Queue an initial fetch of data during attach and after every read
rather than synchronously fetching data and polling for completion.

If data has not been returned from an previous fetch during read,
just return EAGAIN rather than blocking.

Co-authored-by: John Baldwin <jhb@FreeBSD.org>

Reviewed by:	markj
Differential Revision:	https://reviews.freebsd.org/D41656

(cherry picked from commit f1c5a2e3a625053e2b70d5b1777d849a4d9328f2)

1 files changed, 43 insertions, 30 deletions

diff --git a/sys/dev/virtio/random/virtio_random.c b/sys/dev/virtio/random/virtio_random.c
index c02b5c98cece..d54e2e6b70d4 100644
--- a/sys/dev/virtio/random/virtio_random.c
+++ b/sys/dev/virtio/random/virtio_random.c
@@ -54,6 +54,8 @@ struct vtrnd_softc {
 	struct virtqueue	*vtrnd_vq;
 	eventhandler_tag	 eh;
 	bool			 inactive;
+	struct sglist		 *vtrnd_sg;
+	uint32_t		 *vtrnd_value;
 };
 
 static int	vtrnd_modevent(module_t, int, void *);
@@ -67,6 +69,7 @@ static int	vtrnd_negotiate_features(struct vtrnd_softc *);
 static int	vtrnd_setup_features(struct vtrnd_softc *);
 static int	vtrnd_alloc_virtqueue(struct vtrnd_softc *);
 static int	vtrnd_harvest(struct vtrnd_softc *, void *, size_t *);
+static void	vtrnd_enqueue(struct vtrnd_softc *sc);
 static unsigned	vtrnd_read(void *, unsigned);
 
 #define VTRND_FEATURES	0
@@ -138,12 +141,17 @@ static int
 vtrnd_attach(device_t dev)
 {
 	struct vtrnd_softc *sc, *exp;
+	size_t len;
 	int error;
 
 	sc = device_get_softc(dev);
 	sc->vtrnd_dev = dev;
 	virtio_set_feature_desc(dev, vtrnd_feature_desc);
 
+	len = sizeof(*sc->vtrnd_value) * HARVESTSIZE;
+	sc->vtrnd_value = malloc_aligned(len, len, M_DEVBUF, M_WAITOK);
+	sc->vtrnd_sg = sglist_build(sc->vtrnd_value, len, M_WAITOK);
+
 	error = vtrnd_setup_features(sc);
 	if (error) {
 		device_printf(dev, "cannot setup features\n");
@@ -174,6 +182,8 @@ vtrnd_attach(device_t dev)
 	sc->inactive = false;
 	random_source_register(&random_vtrnd);
 
+	vtrnd_enqueue(sc);
+
 fail:
 	if (error)
 		vtrnd_detach(dev);
@@ -185,6 +195,7 @@ static int
 vtrnd_detach(device_t dev)
 {
 	struct vtrnd_softc *sc;
+	uint32_t rdlen;
 
 	sc = device_get_softc(dev);
 	KASSERT(
@@ -197,7 +208,13 @@ vtrnd_detach(device_t dev)
 		sc->eh = NULL;
 	}
 	random_source_deregister(&random_vtrnd);
+
+	/* clear the queue */
+	virtqueue_poll(sc->vtrnd_vq, &rdlen);
+
 	atomic_store_explicit(&g_vtrnd_softc, NULL, memory_order_release);
+	sglist_free(sc->vtrnd_sg);
+	zfree(sc->vtrnd_value, M_DEVBUF);
 	return (0);
 }
 
@@ -251,49 +268,45 @@ vtrnd_alloc_virtqueue(struct vtrnd_softc *sc)
 	return (virtio_alloc_virtqueues(dev, 0, 1, &vq_info));
 }
 
+static void
+vtrnd_enqueue(struct vtrnd_softc *sc)
+{
+	struct virtqueue *vq;
+	int error __diagused;
+
+	vq = sc->vtrnd_vq;
+
+	KASSERT(virtqueue_empty(vq), ("%s: non-empty queue", __func__));
+
+	error = virtqueue_enqueue(vq, sc, sc->vtrnd_sg, 0, 1);
+	KASSERT(error == 0, ("%s: virtqueue_enqueue returned error: %d",
+	    __func__, error));
+
+	virtqueue_notify(vq);
+}
+
 static int
 vtrnd_harvest(struct vtrnd_softc *sc, void *buf, size_t *sz)
 {
-	struct sglist_seg segs[1];
-	struct sglist sg;
 	struct virtqueue *vq;
-	uint32_t value[HARVESTSIZE] __aligned(sizeof(uint32_t) * HARVESTSIZE);
+	void *cookie;
 	uint32_t rdlen;
-	int error;
-
-	_Static_assert(sizeof(value) < PAGE_SIZE, "sglist assumption");
 
 	if (sc->inactive)
 		return (EDEADLK);
 
-	sglist_init(&sg, 1, segs);
-	error = sglist_append(&sg, value, *sz);
-	if (error != 0)
-		panic("%s: sglist_append error=%d", __func__, error);
-
 	vq = sc->vtrnd_vq;
-	KASSERT(virtqueue_empty(vq), ("%s: non-empty queue", __func__));
-
-	error = virtqueue_enqueue(vq, buf, &sg, 0, 1);
-	if (error != 0)
-		return (error);
-
-	/*
-	 * Poll for the response, but the command is likely already
-	 * done when we return from the notify.
-	 */
-	virtqueue_notify(vq);
-	virtqueue_poll(vq, &rdlen);
 
-	if (rdlen > *sz)
-		panic("%s: random device wrote %zu bytes beyond end of provided"
-		    " buffer %p:%zu", __func__, (size_t)rdlen - *sz,
-		    (void *)value, *sz);
-	else if (rdlen == 0)
+	cookie = virtqueue_dequeue(vq, &rdlen);
+	if (cookie == NULL)
 		return (EAGAIN);
+	KASSERT(cookie == sc, ("%s: cookie mismatch", __func__));
+
 	*sz = MIN(rdlen, *sz);
-	memcpy(buf, value, *sz);
-	explicit_bzero(value, *sz);
+	memcpy(buf, sc->vtrnd_value, *sz);
+
+	vtrnd_enqueue(sc);
+
 	return (0);
 }