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You can consult this document which explain in details the concept of SL/VL & QOS (QoS Architecture) based on our IB Spec.
Our Mellanox OFED UM has a section as well on QoS Architecture (Quality Of Service).
By default, all traffic is being sent to SL0 (one buffer) if QOS is not being implemented.
I read this article. Good article. But I want to clarify. Is VL have dedicated buffers or its like shared register file with ranges between VLs. Is VL buffers idle if I use only 1 VL ?
Now I think I found Indirect answer about this in presentation  (slide 37, 4.2 Study of switch buffer size).
In this presentation study dedicated VLs buffers. Its give me indirect answer that each VLs have dedicated/own buffers and that is I use one SL/VL than others VL is Idle.
1 Analyzing InfiniBand Packets https://www.openfabrics.org/images/eventpresos/workshops2015/UGWorkshop/Thursday/thursday_09.pdf
Quick answer to your question: Yes. VL has a dedicated/pre-allocated buffers.