From ad5f4fe4236fa6fc436619a2c53d8ebc51bec4ae Mon Sep 17 00:00:00 2001 From: Ian Jackson Date: Thu, 28 Jun 2012 18:43:21 +0100 Subject: libxl: domain save/restore: run in a separate process libxenctrl expects to be able to simply run the save or restore operation synchronously. This won't work well in a process which is trying to handle multiple domains. The options are: - Block such a whole process (eg, the whole of libvirt) while migration completes (or until it fails). - Create a thread to run xc_domain_save and xc_domain_restore on. This is quite unpalatable. Multithreaded programming is error prone enough without generating threads in libraries, particularly if the thread does some very complex operation. - Fork and run the operation in the child without execing. This is no good because we would need to negotiate with the caller about fds we would inherit (and we might be a very large process). - Fork and exec a helper. Of these options the latter is the most palatable. Consequently: * A new helper program libxl-save-helper (which does both save and restore). It will be installed in /usr/lib/xen/bin. It does not link against libxl, only libxc, and its error handling does not need to be very advanced. It does contain a plumbing through of the logging interface into the callback stream. * A small ad-hoc protocol between the helper and libxl which allows log messages and the libxc callbacks to be passed up and down. Protocol doc comment is in libxl_save_helper.c. * To avoid a lot of tedium the marshalling boilerplate (stubs for the helper and the callback decoder for libxl) is generated with a small perl script. * Implement new functionality to spawn the helper, monitor its output, provide responses, and check on its exit status. * The functions libxl__xc_domain_restore_done and libxl__xc_domain_save_done now turn out to want be called in the same place. So make their state argument a void* so that the two functions are type compatible. The domain save path still writes the qemu savefile synchronously. This will need to be fixed in a subsequent patch. Signed-off-by: Ian Jackson Acked-by: Ian Campbell --- .gitignore | 1 + 1 file changed, 1 insertion(+) (limited to '.gitignore') diff --git a/.gitignore b/.gitignore index 24b9846e50..0891d903c4 100644 --- a/.gitignore +++ b/.gitignore @@ -355,6 +355,7 @@ tools/libxl/_*.[ch] tools/libxl/testidl tools/libxl/testidl.c tools/libxl/*.pyc +tools/libxl/libxl-save-helper tools/blktap2/control/tap-ctl tools/firmware/etherboot/eb-roms.h tools/firmware/etherboot/gpxe-git-snapshot.tar.gz -- cgit v1.2.3