* Steven Sistare (steven.sist...@oracle.com) wrote: > On 6/3/2021 4:44 PM, Daniel P. Berrangé wrote: > > On Thu, Jun 03, 2021 at 08:36:42PM +0100, Dr. David Alan Gilbert wrote: > >> * Steven Sistare (steven.sist...@oracle.com) wrote: > >>> On 5/24/2021 6:39 AM, Dr. David Alan Gilbert wrote: > >>>> * Steven Sistare (steven.sist...@oracle.com) wrote: > >>>>> On 5/20/2021 9:13 AM, Dr. David Alan Gilbert wrote: > >>>>>> On the 'restart' branch of questions; can you explain, > >>>>>> other than the passing of the fd's, why the outgoing side of > >>>>>> qemu's 'migrate exec:' doesn't work for you? > >>>>> > >>>>> I'm not sure what I should describe. Can you be more specific? > >>>>> Do you mean: can we add the cpr specific bits to the migrate exec code? > >>>> > >>>> Yes; if possible I'd prefer to just keep the one exec mechanism. > >>>> It has an advantage of letting you specify the new command line; that > >>>> avoids the problems I'd pointed out with needing to change the command > >>>> line if a hotplug had happened. It also means we only need one chunk of > >>>> exec code. > >>> > >>> How/where would you specify a new command line? Are you picturing the > >>> usual migration > >>> setup where you start a second qemu with its own arguments, plus a > >>> migrate_incoming > >>> option or command? That does not work for live update restart; the old > >>> qemu must exec > >>> the new qemu. Or something else? > >> > >> The existing migration path allows an exec - originally intended to exec > >> something like a compressor or a store to file rather than a real > >> migration; i.e. you can do: > >> > >> migrate "exec:gzip > mig" > >> > >> and that will save the migration stream to a compressed file called mig. > >> Now, I *think* we can already do: > >> > >> migrate "exec:path-to-qemu command line parameters -incoming 'hmmmmm'" > >> (That's probably cleaner via the QMP interface). > >> > >> I'm not quite sure what I want in the incoming there, but that is > >> already the source execing the destination qemu - although I think we'd > >> probably need to check if that's actually via an intermediary. > > > > I don't think you can dirctly exec qemu in that way, because the > > source QEMU migration code is going to wait for completion of the > > QEMU you exec'd and that'll never come on success. So you'll end > > up with both QEMU's running forever. If you pass the daemonize > > option to the new QEMU then it will immediately detach itself, > > and the source QEMU will think the migration command has finished > > or failed. > > > > I think you can probably do it if you use a wrapper script though. > > The wrapper would have to fork QEMU in the backend, and then the > > wrapper would have to monitor the new QEMU to see when the incoming > > migration has finished/aborted, at which point the wrapper can > > exit, so the source QEMU sees a successful cleanup of the exec'd > > command. </hand waving> > > cpr restart does not work for any scheme that involves the old qemu process > co-existing with > the new qemu process. To preserve descriptors and anonymous memory, cpr > restart requires > that old qemu directly execs new qemu. Not fork-exec. Same pid. > > So responding to Dave's comment, "keep the one exec mechanism", that is not > possible. > We still need the qemu_exec_requested mechanism to cause a direct exec after > state is > saved.
OK, note if you can find anyway to make kernel changes to avoid this kexec, life is going to get *much* better; starting a separate qemu at the management layer would be so much easier. > >>> We could shoehorn cpr restart into the migrate exec path by defining a > >>> new migration > >>> capability that the client would set before issuing the migrate command. > >>> However, the > >>> implementation code would be sprinkled with conditionals to suppress > >>> migrate-only bits > >>> and call cpr-only bits. IMO that would be less maintainable than having > >>> a separate > >>> cprsave function. Currently cprsave does not duplicate any migration > >>> functionality. > >>> cprsave calls qemu_save_device_state() which is used by xen. > >> > >> To me it feels like cprsave in particular is replicating more code. > >> > >> It's also jumping through hoops in places to avoid changing the > >> commandline; that's going to cause more pain for a lot of people - not > >> just because it's hacks all over for that, but because a lot of people > >> are actually going to need to change the commandline even in a cpr like > >> case (e.g. due to hotplug or changing something else about the > >> environment, like auth data or route to storage or networking that > >> changed). > > > > Management apps that already support migration, will almost certainly > > know how to start up a new QEMU with a different command line that > > takes account of hotplugged/unplugged devices. IOW avoiding changing > > the command line only really addresses the simple case, and the hard > > case is likely already solved for purposes of handling regular live > > migration. > > Agreed, with the caveat that for cpr, the management app must communicate the > new arguments > to the qemu-exec trampoline, rather than passing the args on the command line > to a new > qemu process. > > >> There are hooks for early parameter parsing, so if we need to add extra > >> commandline args we can; but for example the case of QEMU_START_FREEZE > >> to add -S just isn't needed as soon as you let go of the idea of needing > >> an identical commandline. > > I'll delete QEMU_START_FREEZE. > > I still need to preserve argv_main and pass it to the qemu-exec trampoline, > though, as > the args contain identifying information that the management app needs to > modify the > arguments based the the instances's hot plug history. > > Or, here is another possibility. We could redefine cprsave to leave the VM > in a > stopped state, and add a cprstart command to be called subsequently that > performs > the exec. It takes a single string argument: a command plus arguments to > exec. > The command may be qemu or a trampoline like qemu-exec. I like that the > trampoline > name is no longer hardcoded. The management app can derive new qemu args for > the > instances as it would with migration, and pass them to the command, instead > of passing > them to qemu-exec via some side channel. cprload finishes the job and does > not change. > I already like this scheme better. Right, that's sounding better; now the other benefit you get is you don't need to play with environment variables; you can define a command line option that takes all the extra data it needs. Dave > - Steve > -- Dr. David Alan Gilbert / dgilb...@redhat.com / Manchester, UK