The NVMe standard (1.4) states
8.6 Doorbell Stride for Software Emulation
The doorbell stride,...is useful in software emulation of an NVM
Express controller. ... For hardware implementations of the NVM
Express interface, the expected doorbell stride value is 0h.
However, hardware in the wild exists with a doorbell stride of
1h. This change supports that hardware, as well as software emulators
as envisioned in Section 8.6. Since this is the fast path, care has
been taken to make this computation efficient. The bit of math to
compute an offset for each is replaced by a memory load from cache.