This commit introduces support for the HMB feature and will setup the
buffer during start-up. The host-memory-buffer (HMB) feature is mostly
used on NVMe devices that do not make use of an DRAM cache to store its
translation tables amongst other operational data. Not using HMB can
impair the performance on such devices.
The memory is allocated in 2 MiB chunks of DMA-capable memory and its
total size in bytes is configurable via the 'hmb_size' config attribute.
The driver always checks the minimal and preferred size of the HMB and
issues a warning in case it is not enabled via the configuration.
Moreover, if the configured size is less than the minimal amount
required by the device the HMB is not configured at all and a warning
is issued also. If the configured size is more than the preferred size
it will be capped to that amount.
Fixes#4715.
ae0e0c118e