os-posix: asynchronous teardown for shutdown on Linux

os-posix: asynchronous teardown for shutdown on Linux

This patch adds support for asynchronously tearing down a VM on Linux.

When qemu terminates, either naturally or because of a fatal signal,
the VM is torn down. If the VM is huge, it can take a considerable
amount of time for it to be cleaned up. In case of a protected VM, it
might take even longer than a non-protected VM (this is the case on
s390x, for example).

Some users might want to shut down a VM and restart it immediately,
without having to wait. This is especially true if management
infrastructure like libvirt is used.

This patch implements a simple trick on Linux to allow qemu to return
immediately, with the teardown of the VM being performed
asynchronously.

If the new commandline option -async-teardown is used, a new process is
spawned from qemu at startup, using the clone syscall, in such way that
it will share its address space with qemu.The new process will have the
name "cleanup/<QEMU_PID>". It will wait until qemu terminates
completely, and then it will exit itself.

This allows qemu to terminate quickly, without having to wait for the
whole address space to be torn down. The cleanup process will exit
after qemu, so it will be the last user of the address space, and
therefore it will take care of the actual teardown. The cleanup
process will share the same cgroups as qemu, so both memory usage and
cpu time will be accounted properly.

If possible, close_range will be used in the cleanup process to close
all open file descriptors. If it is not available or if it fails, /proc
will be used to determine which file descriptors to close.

If the cleanup process is forcefully killed with SIGKILL before the
main qemu process has terminated completely, the mechanism is defeated
and the teardown will not be asynchronous.

This feature can already be used with libvirt by adding the following
to the XML domain definition to pass the parameter to qemu directly:

  <commandline xmlns="http://libvirt.org/schemas/domain/qemu/1.0">
  <arg value='-async-teardown'/>
  </commandline>

Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
Reviewed-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Tested-by: Murilo Opsfelder Araujo <muriloo@linux.ibm.com>
Message-Id: <20220812133453.82671-1-imbrenda@linux.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>