Until now, destroying a protected guest was an entirely synchronous
operation that could potentially take a very long time, depending on
the size of the guest, due to the time needed to clean up the address
space from protected pages.

This patch implements an asynchronous destroy mechanism, that allows a
protected guest to reboot significantly faster than previously.

This is achieved by clearing the pages of the old guest in background.
In case of reboot, the new guest will be able to run in the same
address space almost immediately.

The old protected guest is then only destroyed when all of its memory
has been destroyed or otherwise made non protected.

Two new PV commands are added for the KVM_S390_PV_COMMAND ioctl:

KVM_PV_ASYNC_CLEANUP_PREPARE: set aside the current protected VM for
later asynchronous teardown. The current KVM VM will then continue
immediately as non-protected. If a protected VM had already been
set aside for asynchronous teardown, but without starting the teardown
process, this call will fail. There can be at most one VM set aside at
any time. Once it is set aside, the protected VM only exists in the
context of the Ultravisor, it is not associated with the KVM VM
anymore. Its protected CPUs have already been destroyed, but not its
memory. This command can be issued again immediately after starting
KVM_PV_ASYNC_CLEANUP_PERFORM, without having to wait for completion.

KVM_PV_ASYNC_CLEANUP_PERFORM: tears down the protected VM previously
set aside using KVM_PV_ASYNC_CLEANUP_PREPARE. Ideally the
KVM_PV_ASYNC_CLEANUP_PERFORM PV command should be issued by userspace
from a separate thread. If a fatal signal is received (or if the
process terminates naturally), the command will terminate immediately
without completing. All protected VMs whose teardown was interrupted
will be put in the need_cleanup list. The rest of the normal KVM
teardown process will take care of properly cleaning up all remaining
protected VMs, including the ones on the need_cleanup list.
Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Nico Boehr <nrb@linux.ibm.com>
Reviewed-by: Janosch Frank <frankja@linux.ibm.com>
Reviewed-by: Steffen Eiden <seiden@linux.ibm.com>
Link: https://lore.kernel.org/r/20221111170632.77622-2-imbrenda@linux.ibm.com
Message-Id: <20221111170632.77622-2-imbrenda@linux.ibm.com>
Signed-off-by: Janosch Frank <frankja@linux.ibm.com>