Re: [PATCH 00/33] migration: capture error reports into Error object

[Dr. David Alan Gilbert]

* Daniel P. Berrangé (berrange@redhat.com) wrote:
> On Mon, Feb 08, 2021 at 01:29:03PM +0000, Dr. David Alan Gilbert wrote:
> > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > > On Thu, Feb 04, 2021 at 06:22:49PM +0000, Dr. David Alan Gilbert wrote:
> > > > * Daniel P. Berrangé (berrange@redhat.com) wrote:
> > > > > Due to its long term heritage most of the migration code just invokes
> > > > > 'error_report' when problems hit. This was fine for HMP, since the
> > > > > messages get redirected from stderr, into the HMP console. It is not
> > > > > OK for QMP because the errors will not be fed back to the QMP client.
> > > > > 
> > > > > This wasn't a terrible real world problem with QMP so far because
> > > > > live migration happens in the background, so at least on the target 
> > > > > side
> > > > > there is not a QMP command that needs to capture the incoming 
> > > > > migration.
> > > > > It is a problem on the source side but it doesn't hit frequently as 
> > > > > the
> > > > > source side has fewer failure scenarios. None the less on both sides 
> > > > > it
> > > > > would be desirable if 'query-migrate' can report errors correctly.
> > > > > With the introduction of the load-snapshot QMP commands, the need for
> > > > > error reporting becomes more pressing.
> > > > > 
> > > > > Wiring up good error reporting is a large and difficult job, which
> > > > > this series does NOT complete. The focus here has been on converting
> > > > > all methods in savevm.c which have an 'int' return value capable of
> > > > > reporting errors. This covers most of the infrastructure for 
> > > > > controlling
> > > > > the migration state serialization / protocol.
> > > > > 
> > > > > The remaining part that is missing error reporting are the callbacks 
> > > > > in
> > > > > the VMStateDescription struct which can return failure codes, but have
> > > > > no "Error **errp" parameter. Thinking about how this might be dealt 
> > > > > with
> > > > > in future, a big bang conversion is likely non-viable. We'll probably
> > > > > want to introduce a duplicate set of callbacks with the "Error **errp"
> > > > > parameter and convert impls in batches, eventually removing the
> > > > > original callbacks. I don't intend todo that myself in the immediate
> > > > > future.
> > > > > 
> > > > > IOW, this patch series probably solves 50% of the problem, but we
> > > > > still do need the rest to get ideal error reporting.
> > > > > 
> > > > > In doing this savevm conversion I noticed a bunch of places which
> > > > > see and then ignore errors. I only fixed one or two of them which
> > > > > were clearly dubious. Other places in savevm.c where it seemed it
> > > > > was probably ok to ignore errors, I've left using error_report()
> > > > > on the basis that those are really warnings. Perhaps they could
> > > > > be changed to warn_report() instead.
> > > > > 
> > > > > There are alot of patches here, but I felt it was easier to review
> > > > > for correctness if I converted 1 function at a time. The series
> > > > > does not neccessarily have to be reviewed/appied in 1 go.
> > > > 
> > > > After this series, what do my errors look like, and where do they end
> > > > up?
> > > > Do I get my nice backtrace shwoing that device failed, then that was
> > > > part of that one...
> > > 
> > > It hasn't modified any of the VMStateDescription callbacks so any
> > > of the per-device logic that was printing errors will still be using
> > > error_report to the console as before.
> > > 
> > > The errors that have changed (at this stage) are only the higher
> > > level ones that are in the generic part of the code. Where those
> > > errors mentioned a device name/ID they still do.
> > > 
> > > In some of the parts I've modified there will have been multiple
> > > error_reports collapsed into one error_setg() but the ones that
> > > are eliminated are high level generic messages with no useful
> > > info, so I don't think loosing those is a problem per-se.
> > > 
> > > The example that I tested was the case where we load a snapshot
> > > under a different config that we saved it with. This is the scenario
> > > that gave the non-deterministic ordering in the iotest you disabled
> > > from my previous series.
> > > 
> > > In that case, we changed from:
> > > 
> > >   qemu-system-x86_64: Unknown savevm section or instance 
> > > '0000:00:02.0/virtio-rng' 0. Make sure that your current VM setup matches 
> > > your saved VM setup, including any hotplugged devices
> > >   {"return": [{"current-progress": 1, "status": "concluded", 
> > > "total-progress": 1, "type": "snapshot-load", "id": "load-err-stderr", 
> > > "error": "Error -22 while loading VM state"}]}
> > > 
> > > To
> > > 
> > >   {"return": [{"current-progress": 1, "status": "concluded", 
> > > "total-progress": 1, "type": "snapshot-load", "id": "load-err-stderr", 
> > > "error": "Unknown savevm section or instance '0000:00:02.0/virtio-rng' 0. 
> > > Make sure that your current VM setup matches your saved VM setup, 
> > > including any hotplugged devices"}]}
> > > 
> > > From a HMP loadvm POV, this means instead of seeing
> > > 
> > >   (hmp)  loadvm foo
> > >   Unknown savevm section or instance '0000:00:02.0/virtio-rng' 0. Make 
> > > sure that your current VM setup matches your saved VM setup, including 
> > > any hotplugged devices
> > >   Error -22 while loading VM state
> > > 
> > > You will only see the detailed error message
> > > 
> > >   (hmp)  loadvm foo
> > >   Unknown savevm section or instance '0000:00:02.0/virtio-rng' 0. Make 
> > > sure that your current VM setup matches your saved VM setup, including 
> > > any hotplugged devices
> > > 
> > > In this case I think loosing the "Error -22 while loading VM state"
> > > is fine, as it didn't add value IMHO.
> > > 
> > > 
> > > If we get around to converting the VMStateDescription callbacks to
> > > take an error object, then I think we'll possibly need to stack the
> > > error message from the callback, with the higher level message.
> > > 
> > > Do you have any familiar/good examples of error message stacking I
> > > can look at ?  I should be able to say whether they would be impacted
> > > by this series or not - if they are, then I hopefully only threw away
> > > the fairly useless high level messages, like the "Error -22" message
> > > above.
> > 
> > Can you try migrating:
> >   ./x86_64-softmmu/qemu-system-x86_64 -M pc -nographic -device 
> > virtio-rng,disable-modern=true
> > to
> >   ./x86_64-softmmu/qemu-system-x86_64 -M pc -nographic -device virtio-rng
> > 
> > what I currently get is:
> > qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x6 read: 0 
> > device: 10 cmask: 10 wmask: 0 w1cmask:0
> > qemu-system-x86_64: Failed to load PCIDevice:config
> > qemu-system-x86_64: Failed to load virtio-rng:virtio
> > qemu-system-x86_64: error while loading state for instance 0x0 of device 
> > '0000:00:04.0/virtio-rng'
> > qemu-system-x86_64: load of migration failed: Invalid argument
> 
> After my patches the very last line is gone.
> 
> So, still reporting using  error_report() is the first 3:
> 
>  qemu-system-x86_64: get_pci_config_device: Bad config data: i=0x6 read: 0 
> device: 10 cmask: 10 wmask: 0 w1cmask:0
>  qemu-system-x86_64: Failed to load PCIDevice:config
>  qemu-system-x86_64: Failed to load virtio-rng:virtio

So those are still ending up in the stderr/log ?

> Then reported in process_incoming_migration_co() using the message
> populated in the Error object, using error_report_err():
> 
>  qemu-system-x86_64: error while loading state for instance 0x0 of device 
> '0000:00:04.0/virtio-rng'

Does that mean we've not got that error associated with the others?  It
could be a pain where we've got multiple devices (e.g. NICs or storage)
and need to realise which one is failing.

> Finally, this is no longer reported:
> 
>  qemu-system-x86_64: load of migration failed: Invalid argument
> 
> So in this case we've not lost any useful information

You occasionally get other things other than Invalid argument; in
particular you get EIO; it can help you determine if the source killed
the migration connection first.

Dave

> Regards,
> Daniel
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-- 
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK