Migration mode is a VM attribute which enables tracking of changes in
storage attributes (PGSTE). It assumes dirty tracking is enabled on all
memslots to keep a dirty bitmap of pages with changed storage attributes.

When enabling migration mode, we currently check that dirty tracking is
enabled for all memslots. However, userspace can disable dirty tracking
without disabling migration mode.

Since migration mode is pointless with dirty tracking disabled, disable
migration mode whenever userspace disables dirty tracking on any slot.

Also update the documentation to clarify that dirty tracking must be
enabled when enabling migration mode, which is already enforced by the
code in kvm_s390_vm_start_migration().

Also highlight in the documentation for KVM_S390_GET_CMMA_BITS that it
can now fail with -EINVAL when dirty tracking is disabled while
migration mode is on. Move all the error codes to a table so this stays
readable.

To disable migration mode, slots_lock should be held, which is taken
in kvm_set_memory_region() and thus held in
kvm_arch_prepare_memory_region().

Restructure the prepare code a bit so all the sanity checking is done
before disabling migration mode. This ensures migration mode isn't
disabled when some sanity check fails.

Cc: stable@vger.kernel.org
Fixes: 190df4a2 ("KVM: s390: CMMA tracking, ESSA emulation, migration mode")
Signed-off-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Link: https://lore.kernel.org/r/20230127140532.230651-2-nrb@linux.ibm.com
Message-Id: <20230127140532.230651-2-nrb@linux.ibm.com>
[frankja@linux.ibm.com: fixed commit message typo, moved api.rst error table upwards]
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>