Re: [PATCH v5 2/2] migration: Update error description outside migration.c

[Tejus GK <tejus.gk@nutanix.com>]

On 03/10/23 6:14 pm, Juan Quintela wrote:
> Tejus GK <tejus.gk@nutanix.com> wrote:
>> A few code paths exist in the source code,where a migration is
>> marked as failed via MIGRATION_STATUS_FAILED, but the failure happens
>> outside	of migration.c
>>
>> In such	cases, an error_report() call is made, however the current
>> MigrationState is never	updated	with the error description, and	hence
>> clients	like libvirt never know	the actual reason for the failure.
>>
>> This patch covers such cases outside of	migration.c and	updates	the
>> error description at the appropriate places.
>>
>> Acked-by: Peter Xu <peterx@redhat.com>
>> Signed-off-by: Tejus GK <tejus.gk@nutanix.com>
> 
> Reviewed-by: Juan Quintela <quintela@redhat.com>
> 
> Queued.
Thanks, will be sending out a patch with the "Reviewed by" trailer added.
> 
> But I wonder.
> 
>> index 1f65294bf4..60eec7c31f 100644
>> --- a/migration/savevm.c
>> +++ b/migration/savevm.c
>> @@ -979,6 +979,8 @@ static void save_section_footer(QEMUFile *f, SaveStateEntry *se)
>>   static int vmstate_save(QEMUFile *f, SaveStateEntry *se, JSONWriter *vmdesc)
>>   {
>>       int ret;
>> +    Error *local_err = NULL;
>> +    MigrationState *s = migrate_get_current();
>>   
>>       if ((!se->ops || !se->ops->save_state) && !se->vmsd) {
>>           return 0;
>> @@ -1002,6 +1004,8 @@ static int vmstate_save(QEMUFile *f, SaveStateEntry *se, JSONWriter *vmdesc)
>>       } else {
>>           ret = vmstate_save_state_with_err(f, se->vmsd, se->opaque, vmdesc, &local_err);
>>           if (ret) {
>> +            migrate_set_error(s, local_err);
>> +            error_report_err(local_err);
> 
> We are setting the error and reporting it.
> 
>>               return ret;
>>           }
>>       }
>> @@ -1068,10 +1072,14 @@ void qemu_savevm_send_open_return_path(QEMUFile *f)
>>   int qemu_savevm_send_packaged(QEMUFile *f, const uint8_t *buf, size_t len)
>>   {
>>       uint32_t tmp;
>> +    MigrationState *ms = migrate_get_current();
>> +    Error *local_err = NULL;
>>   
>>       if (len > MAX_VM_CMD_PACKAGED_SIZE) {
>> -        error_report("%s: Unreasonably large packaged state: %zu",
>> +        error_setg(&local_err, "%s: Unreasonably large packaged state: %zu",
>>                        __func__, len);
>> +        migrate_set_error(ms, local_err);
>> +        error_report_err(local_err);
> 
> Again we set the error and we report it.
> 
>>           return -1;
>>       }
>>   
>> @@ -1499,8 +1507,11 @@ int qemu_savevm_state_complete_precopy_non_iterable(QEMUFile *f,
>>            * bdrv_activate_all() on the other end won't fail. */
>>           ret = bdrv_inactivate_all();
>>           if (ret) {
>> -            error_report("%s: bdrv_inactivate_all() failed (%d)",
>> -                         __func__, ret);
>> +            Error *local_err = NULL;
>> +            error_setg(&local_err, "%s: bdrv_inactivate_all() failed (%d)",
>> +                       __func__, ret);
>> +            migrate_set_error(ms, local_err);
>> +            error_report_err(local_err);
> 
> Again.
> 
>>               qemu_file_set_error(f, ret);
> 
> And we still have qemu_file_set_error() here, ouch.
> 
>>               return ret;
>>           }
>> diff --git a/migration/vmstate.c b/migration/vmstate.c
>> index dd9c76dbeb..4cde30bf2d 100644
>> --- a/migration/vmstate.c
>> +++ b/migration/vmstate.c
>> @@ -14,6 +14,7 @@
>>   #include "migration.h"
>>   #include "migration/vmstate.h"
>>   #include "savevm.h"
>> +#include "qapi/error.h"
>>   #include "qapi/qmp/json-writer.h"
>>   #include "qemu-file.h"
>>   #include "qemu/bitops.h"
>> @@ -336,7 +337,7 @@ int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>>           ret = vmsd->pre_save(opaque);
>>           trace_vmstate_save_state_pre_save_res(vmsd->name, ret);
>>           if (ret) {
>> -            error_report("pre-save failed: %s", vmsd->name);
>> +            error_setg(errp, "pre-save failed: %s", vmsd->name);
> 
> Here we only set the error
> 
>>               return ret;
>>           }
>>       }
>> @@ -389,8 +390,8 @@ int vmstate_save_state_v(QEMUFile *f, const VMStateDescription *vmsd,
>>                                        vmdesc_loop);
>>                   }
>>                   if (ret) {
>> -                    error_report("Save of field %s/%s failed",
>> -                                 vmsd->name, field->name);
>> +                    error_setg(errp, "Save of field %s/%s failed",
>> +                                vmsd->name, field->name);
> 
> Same here.
You're right, I'm only setting it here and reporting it eventually in 
savevm.c. The trivial solution for this would have been directly doing a 
migrate_set_error() here, but that ended up breaking the build for the 
unit test test-vmstate.c
> 
>>                       if (vmsd->post_save) {
>>                           vmsd->post_save(opaque);
>>                       }
> 
> 
> So, I am wondering if it could be better to just report the error in a
> single place for migration, and set it whenever we need it?
Yes, that would be very convenient, for all the errors to be reported in 
lets say migration.c. Though that'd also require all the subsystems 
under migration.c to properly propagate the errors.
> 
> That is independent of this patch, though.
> 
> Later, Juan.
> 

regards,
tejus