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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>