* Kevin Wolf (kw...@redhat.com) wrote: > Am 13.05.2020 um 12:53 hat Dr. David Alan Gilbert geschrieben: > > * Kevin Wolf (kw...@redhat.com) wrote: > > > Am 12.05.2020 um 11:43 hat Daniel P. Berrangé geschrieben: > > > > On Tue, May 12, 2020 at 11:32:06AM +0200, Lukas Straub wrote: > > > > > On Mon, 11 May 2020 16:46:45 +0100 > > > > > "Dr. David Alan Gilbert" <dgilb...@redhat.com> wrote: > > > > > > > > > > > * Daniel P. Berrangé (berra...@redhat.com) wrote: > > > > > > > ... > > > > > > > That way if QEMU does get stuck, you can start by tearing down the > > > > > > > least distruptive channel. eg try tearing down the migration > > > > > > > connection > > > > > > > first (which shouldn't negatively impact the guest), and only if > > > > > > > that > > > > > > > doesn't work then, move on to tear down the NBD connection (which > > > > > > > risks > > > > > > > data loss) > > > > > > > > > > > > I wonder if a different way would be to make all network connections > > > > > > register with yank, but then make yank take a list of connections to > > > > > > shutdown(2). > > > > > > > > > > Good Idea. We could name the connections (/yank callbacks) in the > > > > > form "nbd:<node-name>", "chardev:<chardev-name>" and "migration" > > > > > (and add "netdev:...", etc. in the future). Then make yank take a > > > > > list of connection names as you suggest and silently ignore > > > > > connections > > > > > that don't exist. And maybe even add a 'query-yank' oob command > > > > > returning > > > > > a list of registered connections so the management application can do > > > > > pattern matching if it wants. > > > > > > I'm generally not a big fan of silently ignoring things. Is there a > > > specific requirement to do it in this case, or can management > > > applications be expected to know which connections exist? > > > > > > > Yes, that would make the yank command much more flexible in how it can > > > > be used. > > > > > > > > As an alternative to using formatted strings like this, it could be > > > > modelled more explicitly in QAPI > > > > > > > > { 'struct': 'YankChannels', > > > > 'data': { 'chardev': [ 'string' ], > > > > 'nbd': ['string'], > > > > 'migration': bool } } > > > > > > > > In this example, 'chardev' would accept a list of chardev IDs which > > > > have it enabled, 'nbd' would accept a list of block node IDs which > > > > have it enabled, and migration is a singleton on/off. > > > > > > Of course, it also means that the yank code needs to know about every > > > single object that supports the operation, whereas if you only have > > > strings, the objects could keep registering their connection with a > > > generic function like yank_register_function() in this version. > > > > > > I'm not sure if the additional complexity is worth the benefits. > > > > I tend to agree; although we do have to ensure we either use an existing > > naming scheme (e.g. QOM object names?) or make sure we've got a well > > defined list of prefixes. > > Not everything that has a network connection is a QOM object (in fact, > neither migration nor chardev nor nbd are QOM objects).
Hmm, migrationstate is a qdev object. > I guess it would be nice to have a single namespace for everything in > QEMU, but the reality is that we have a few separate ones. As long as we > consistently add a prefix that identifies the namespace in question, I > think that would work. > This means that if we're using node-name to identify the NBD connection, > the namespace should be 'block' rather than 'nbd'. > > One more thing to consider is, what if a single object has multiple > connections? In the case of node-names, we have a limited set of allowed > characters, so we can use one of the remaining characters as a separator > and then suffix a counter. In other places, the identifier isn't > restricted, so suffixing doesn't work. Maybe prefixing does, but it > would have to be there from the beginning then. Yeh I worry about whether on nbd if you can have multiple nbd connections to one block device. > And another thing: Do we really want to document this as limited to > network connections? Another common cause of hangs is when you have > image files on an NFS mount and the connection goes away. Of course, in > the end this is still networking, but inside of QEMU it looks like > accessing any other file. I'm not sure that we'll allow yanking access > to image files anytime soon, but it might not hurt to keep it at the > back of our mind as a potential option we might want the design to > allow. Yep. Dave > Kevin -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK