According to the virtio v1.x "entropy device" specification, a virtio-rng device is supposed to always return at least one byte of entropy. However the virtio v0.9 spec does not mention such a requirement.
The Arm Fixed Virtual Platform (FVP) implementation of virtio-rng always returns 8 bytes less of entropy than requested. If 8 bytes or less are requested, it will return 0 bytes. This behaviour makes U-Boot's virtio_rng_read() implementation go into an endless loop, hanging the system.
Work around this problem by always requesting 8 bytes more than needed, but only if a previous call to virtqueue_get_buf() returned 0 bytes.
This should never trigger on a v1.x spec compliant implementation, but fixes the hang on the Arm FVP.
Signed-off-by: Andre Przywara andre.przywara@arm.com Reported-by: Peter Hoyes peter.hoyes@arm.com --- drivers/virtio/virtio_rng.c | 9 +++++++-- 1 file changed, 7 insertions(+), 2 deletions(-)
diff --git a/drivers/virtio/virtio_rng.c b/drivers/virtio/virtio_rng.c index b85545c2ee5..786359a6e36 100644 --- a/drivers/virtio/virtio_rng.c +++ b/drivers/virtio/virtio_rng.c @@ -20,7 +20,7 @@ struct virtio_rng_priv { static int virtio_rng_read(struct udevice *dev, void *data, size_t len) { int ret; - unsigned int rsize; + unsigned int rsize = 1; unsigned char buf[BUFFER_SIZE] __aligned(4); unsigned char *ptr = data; struct virtio_sg sg; @@ -29,7 +29,12 @@ static int virtio_rng_read(struct udevice *dev, void *data, size_t len)
while (len) { sg.addr = buf; - sg.length = min(len, sizeof(buf)); + /* + * Work around implementations which always return 8 bytes + * less than requested, down to 0 bytes, which would + * cause an endless loop otherwise. + */ + sg.length = min(rsize ? len : len + 8, sizeof(buf)); sgs[0] = &sg;
ret = virtqueue_add(priv->rng_vq, sgs, 0, 1);