On 6/7/2021 12:40 PM, Steven Sistare 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. > >>>> 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.
Or, pass argv as an additional parameter to cprsave. Daniel, David, do you like passing argv to cprsave or a new cprstart command better than the current scheme? I am ready to sent V4 of the series after we resolve this and the question of whether or not to fold cpr into the migration command. - Steve
