Linux is very conservative with how much entropy it credits to things like RDRAND since they can't be easily trusted. Looks like you get one extra bit per interrupt:

https://github.com/torvalds/linux/blob/master/drivers/char/r...

You'll note no other use of arch_get_random_* throws anything at credit_entropy_bits().

Linux 3.17 did bump up the assumed quality of virtio-rnd:

https://github.com/torvalds/linux/commit/34679ec7a0c45da8161...