MFC r312603: Add initial support for CTL module unloading.

It is only a first step and not perfect, but better then nothing.
The main blocker is CAM target frontend, that can not be unloaded,
since CAM does not have mechanism to unregister periph driver now.

MFC r312291, r312669:
Make CTL frontends report kern_data_resid for under-/overruns.

It seems like kern_data_resid was never really implemented. This change
finally does it. Now frontends update this field while transferring data,
while CTL/backends getting it can more flexibly handle the result.
At this point behavior should not change significantly, still reporting
errors on write overrun, but that may be changed later, if we decide so.

CAM target frontend still does not properly handle overruns due to CAM API
limitations. We may need to add some fields to struct ccb_accept_tio to
pass information about initiator requested transfer size(s).

MFC r310539: Remove CTL_MAX_LUNS from places where it is not required.

MFC r288221: Remove some dead code found by Clang analyzer.

MFC r288020: Remove couple excess SGLIST I/O flags.

Those flags duplicated respective (sg_entries > 0) values.

MFC r287724: Check for obsolete NUL bin in CSCD descriptor.

MFC r287715: Improve XCOPY error reporting.

MFC r287621: Reimplement CTL High Availability.

CTL HA functionality was originally implemented by Copan many years ago,
but large part of the sources was never published. This change includes
clean room implementation of the missing code and fixes for many bugs.

This code supports dual-node HA with ALUA in four modes:
- Active/Unavailable without interlink between nodes;
- Active/Standby with second node handling only basic LUN discovery and
reservation, synchronizing with the first node through the interlink;
- Active/Active with both nodes processing commands and accessing the
backing storage, synchronizing with the first node through the interlink;
- Active/Active with second node working as proxy, transfering all
commands to the first node for execution through the interlink.

Unlike original Copan's implementation, depending on specific hardware,
this code uses simple custom TCP-based protocol for interlink. It has
no authentication, so it should never be enabled on public interfaces.

The code may still need some polishing, but generally it is functional.

Relnotes: yes
Sponsored by: iXsystems, Inc.

MFC r287620: Remove unused target and initiator IDs.

MFC r287372:
Make most of port methods optional and remove bunch of dummies.

MFC r286806: Drop "internal" CTL frontend.

Its idea was to be a simple initiator and execute several commands from
kernel level, but FreeBSD never had consumer for that functionality,
while its implementation polluted many unrelated places.

MFC r284640: Bring per-port LUN enable/disable code up to date:
- remove last remnants of never implemented multiple targets support;
- implement missing support for LUN mapping in this area.

Due to existing locking constraints LUN mapping code is practically
unlocked at this point. Hopefully it is not racy enough to live until
somebody get idea how to call sleeping fronend methods under lock also
taken by the same frontend in non-sleepable context. :(

MFC r278625: Make XCOPY and WUT commands respect physical block size/offset.

This change by 2-3 times improves performance of misaligned XCOPY and WUT
commands by avoiding unneeded read-modify-write cycles inside ZFS.

MFC r278037: CTL LUN mapping rewrite.

Replace iSCSI-specific LUN mapping mechanism with new one, working for any
ports. By default all ports are created without LUN mapping, exposing all
CTL LUNs as before. But, if needed, LUN mapping can be manually set on
per-port basis via ctladm. For its iSCSI ports ctld does it via ioctl(2).
The next step will be to teach ctld to work with FibreChannel ports also.

Respecting additional flexibility of the new mechanism, ctl.conf now allows
alternative syntax for LUN definition. LUNs can now be defined in global
context, and then referenced from targets by unique name, as needed. It
allows same LUN to be exposed several times via multiple targets.

While there, increase limit for LUNs per target in ctld from 256 to 1024.
Some initiators do not support LUNs above 255, but that is not our problem.

Relnotes: yes
Sponsored by: iXsystems, Inc.

MFC r275942: Reduce number of places where global control_softc is used.

At some point we may want to have several CTL instances, and that is not
really impossible.

MFC r274785: Partially reconstruct Active/Standby clusting.

In this mode one head is in Active state, supporting all commands, while
another is in Standby state, supporting only minimal LUN discovery subset.

It is still incomplete since Standby state requires reservation support,
which is impossible to do right without having interlink between heads.
But it allows to run some basic experiments.

MFC r268767:
Add support for VMWare dialect of EXTENDED COPY command, aka VAAI Clone.

This allows to clone VMs and move them between LUNs inside one storage
host without generating extra network traffic to the initiator and back,
and without being limited by network bandwidth.

LUNs participating in copy operation should have UNIQUE NAA or EUI IDs set.
For LUNs without these IDs VMWare will use traditional copy operations.

Beware: the above LUN IDs explicitly set to values non-unique from the VM
cluster point of view may cause data corruption if wrong LUN is addressed!

Sponsored by: iXsystems, Inc.

MFC r288221: Remove some dead code found by Clang analyzer.

MFC r288020: Remove couple excess SGLIST I/O flags.

Those flags duplicated respective (sg_entries > 0) values.

MFC r287724: Check for obsolete NUL bin in CSCD descriptor.

MFC r287715: Improve XCOPY error reporting.

MFC r287621: Reimplement CTL High Availability.

CTL HA functionality was originally implemented by Copan many years ago,
but large part of the sources was never published. This change includes
clean room implementation of the missing code and fixes for many bugs.

This code supports dual-node HA with ALUA in four modes:
- Active/Unavailable without interlink between nodes;
- Active/Standby with second node handling only basic LUN discovery and
reservation, synchronizing with the first node through the interlink;
- Active/Active with both nodes processing commands and accessing the
backing storage, synchronizing with the first node through the interlink;
- Active/Active with second node working as proxy, transfering all
commands to the first node for execution through the interlink.

Unlike original Copan's implementation, depending on specific hardware,
this code uses simple custom TCP-based protocol for interlink. It has
no authentication, so it should never be enabled on public interfaces.

The code may still need some polishing, but generally it is functional.

Relnotes: yes
Sponsored by: iXsystems, Inc.

MFC r287620: Remove unused target and initiator IDs.

MFC r287372:
Make most of port methods optional and remove bunch of dummies.

MFC r286806: Drop "internal" CTL frontend.

Its idea was to be a simple initiator and execute several commands from
kernel level, but FreeBSD never had consumer for that functionality,
while its implementation polluted many unrelated places.

MFC r284640: Bring per-port LUN enable/disable code up to date:
- remove last remnants of never implemented multiple targets support;
- implement missing support for LUN mapping in this area.

Due to existing locking constraints LUN mapping code is practically
unlocked at this point. Hopefully it is not racy enough to live until
somebody get idea how to call sleeping fronend methods under lock also
taken by the same frontend in non-sleepable context. :(

MFC r278625: Make XCOPY and WUT commands respect physical block size/offset.

This change by 2-3 times improves performance of misaligned XCOPY and WUT
commands by avoiding unneeded read-modify-write cycles inside ZFS.

MFC r278037: CTL LUN mapping rewrite.

Replace iSCSI-specific LUN mapping mechanism with new one, working for any
ports. By default all ports are created without LUN mapping, exposing all
CTL LUNs as before. But, if needed, LUN mapping can be manually set on
per-port basis via ctladm. For its iSCSI ports ctld does it via ioctl(2).
The next step will be to teach ctld to work with FibreChannel ports also.

Respecting additional flexibility of the new mechanism, ctl.conf now allows
alternative syntax for LUN definition. LUNs can now be defined in global
context, and then referenced from targets by unique name, as needed. It
allows same LUN to be exposed several times via multiple targets.

While there, increase limit for LUNs per target in ctld from 256 to 1024.
Some initiators do not support LUNs above 255, but that is not our problem.

Relnotes: yes
Sponsored by: iXsystems, Inc.

MFC r275942: Reduce number of places where global control_softc is used.

At some point we may want to have several CTL instances, and that is not
really impossible.

MFC r274785: Partially reconstruct Active/Standby clusting.

In this mode one head is in Active state, supporting all commands, while
another is in Standby state, supporting only minimal LUN discovery subset.

It is still incomplete since Standby state requires reservation support,
which is impossible to do right without having interlink between heads.
But it allows to run some basic experiments.

MFC r268767:
Add support for VMWare dialect of EXTENDED COPY command, aka VAAI Clone.

This allows to clone VMs and move them between LUNs inside one storage
host without generating extra network traffic to the initiator and back,
and without being limited by network bandwidth.

LUNs participating in copy operation should have UNIQUE NAA or EUI IDs set.
For LUNs without these IDs VMWare will use traditional copy operations.

Beware: the above LUN IDs explicitly set to values non-unique from the VM
cluster point of view may cause data corruption if wrong LUN is addressed!

Sponsored by: iXsystems, Inc.