Re: [PATCH V3 00/22] Live Update [restart] : exec

["Daniel P. Berrangé" <berrange@redhat.com>]

On Mon, Jun 14, 2021 at 10:31:32AM -0400, Steven Sistare wrote:
> 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.sistare@oracle.com) wrote:
> >>>> On 5/24/2021 6:39 AM, Dr. David Alan Gilbert wrote:
> >>>>> * Steven Sistare (steven.sistare@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.

I don't really have a strong opinion on this either way.


Regards,
Daniel
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