For some old Linux Guest kernels, the new NVMe 1.3 feature: shadow
doorbell buffer is not enabled, while here, make a dummy BAR region
inside slave target, when Guest submits a new request, the doorbell
value will be write to the shared memory between Guest and vhost
target, so that the existing vhost target can support both new
Linux Guest kernel(newer than 4.12) and old Guest kernel.
Also, the shared BAR space can be used in future which we can move
ADMIN queue processing into SPDK vhost target, with this feature,
the QEMU driver will become very small and easy for upstreaming.
Change-Id: I9463e9f13421368f43bfe4076facddd119f4552e
Signed-off-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-on: https://review.gerrithub.io/419157
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Chandler-Test-Pool: SPDK Automated Test System <sys_sgsw@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Darek Stojaczyk <dariusz.stojaczyk@intel.com>
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