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

From: Dr. David Alan Gilbert
Subject: Re: [PATCH 00/33] migration: capture error reports into Error object
Date: Thu, 4 Feb 2021 18:22:49 +0000
User-agent: Mutt/1.14.6 (2020-07-11)

* 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
Do I get my nice backtrace shwoing that device failed, then that was
part of that one...


> Daniel P. Berrangé (33):
>   migration: push Error **errp into qemu_loadvm_state()
>   migration: push Error **errp into qemu_loadvm_state_header()
>   migration: push Error **errp into qemu_loadvm_state_setup()
>   migration: push Error **errp into qemu_load_device_state()
>   migration: push Error **errp into qemu_loadvm_state_main()
>   migration: push Error **errp into qemu_loadvm_section_start_full()
>   migration: push Error **errp into qemu_loadvm_section_part_end()
>   migration: push Error **errp into loadvm_process_command()
>   migration: push Error **errp into loadvm_handle_cmd_packaged()
>   migration: push Error **errp into loadvm_postcopy_handle_advise()
>   migration: push Error **errp into ram_postcopy_incoming_init()
>   migration: push Error **errp into loadvm_postcopy_handle_listen()
>   migration: push Error **errp into loadvm_postcopy_handle_run()
>   migration: push Error **errp into loadvm_postcopy_ram_handle_discard()
>   migration: make loadvm_postcopy_handle_resume() void
>   migration: push Error **errp into loadvm_handle_recv_bitmap()
>   migration: push Error **errp into loadvm_process_enable_colo()
>   migration: push Error **errp into colo_init_ram_cache()
>   migration: push Error **errp into check_section_footer()
>   migration: push Error **errp into global_state_store()
>   migration: remove error reporting from qemu_fopen_bdrv() callers
>   migration: push Error **errp into qemu_savevm_state_iterate()
>   migration: simplify some error reporting in save_snapshot()
>   migration: push Error **errp into qemu_savevm_state_setup()
>   migration: push Error **errp into qemu_savevm_state_complete_precopy()
>   migration: push Error **errp into
>     qemu_savevm_state_complete_precopy_non_iterable()
>   migration: push Error **errp into qemu_savevm_state_complete_precopy()
>   migration: push Error **errp into qemu_savevm_send_packaged()
>   migration: push Error **errp into qemu_savevm_live_state()
>   migration: push Error **errp into qemu_save_device_state()
>   migration: push Error **errp into qemu_savevm_state_resume_prepare()
>   migration: push Error **errp into postcopy_resume_handshake()
>   migration: push Error **errp into postcopy_do_resume()
>  include/migration/colo.h                      |   2 +-
>  include/migration/global_state.h              |   2 +-
>  migration/colo.c                              |  12 +-
>  migration/global_state.c                      |   6 +-
>  migration/migration.c                         |  80 ++-
>  migration/postcopy-ram.c                      |   8 +-
>  migration/postcopy-ram.h                      |   2 +-
>  migration/ram.c                               |  17 +-
>  migration/ram.h                               |   4 +-
>  migration/savevm.c                            | 594 ++++++++++--------
>  migration/savevm.h                            |  23 +-
>  .../tests/internal-snapshots-qapi.out         |   3 +-
>  12 files changed, 427 insertions(+), 326 deletions(-)
> -- 
> 2.29.2
Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK

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