net/mlx5: control transmit doorbell register mapping

net/mlx5: control transmit doorbell register mapping

The rdma core library can map doorbell register in two ways,
depending on the environment variable "MLX5_SHUT_UP_BF":

  - as regular cached memory, the variable is either missing or
    set to zero. This type of mapping may cause the significant
    doorbell register writing latency and requires explicit
    memory write barrier to mitigate this issue and prevent
    write combining.

  - as non-cached memory, the variable is present and set to
    not "0" value. This type of mapping may cause performance
    impact under heavy loading conditions but the explicit write
    memory barrier is not required and it may improve core
    performance.

The new devarg is introduced "tx_db_nc", if this parameter is
set to zero, the doorbell register is forced to be mapped to
cached memory and requires explicit memory barrier after
writing to. If "tx_db_nc" is set to non-zero value the doorbell
will be mapped as non-cached memory, not requiring the memory
barrier. If "tx_db_nc" is missing the behaviour will be defined
by presence of "MLX5_SHUT_UP_BF" in environment. If variable
is missed the default value zero will be set for ARM64 hosts
and one for others.

In run time the code checks the mapping type and provides the
memory barrier after writing to tx doorbell register if it is
needed. The mapping type is extracted directly from the
uar_mmap_offset field in the queue properties.

Fixes: 18a1c20044c0 ("net/mlx5: implement Tx burst template")
Cc: stable@dpdk.org
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Matan Azrad <matan@mellanox.com>