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

[Daniel P . Berrangé]

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

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'

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

Regards,
Daniel
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