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spapr: Explain purpose of ->fwnmi_migration_blocker more clearly

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spapr_mce_req_event() makes an effort to prevent migration from
degrading the reporting of FWNMIs.  It adds a migration blocker when
it receives one, and deletes it when it's done handling it.  This is a
best effort.

Commit 2500fb423a "migration: Include migration support for machine
check handling" tried to explain this in a comment.  Rewrite the
comment for clarity, and reposition it to make it clear it applies to
all failure modes, not just "migration already in progress".

Cc: David Gibson <david@gibson.dropbear.id.au>
Cc: Aravinda Prasad <arawinda.p@gmail.com>
Cc: Ganesh Goudar <ganeshgr@linux.ibm.com>
Cc: Dr. David Alan Gilbert <dgilbert@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>
Message-Id: <20210720125408.387910-4-armbru@redhat.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
This commit is contained in:
Markus Armbruster 2021-07-20 14:53:55 +02:00
parent e2ef4fc7ae
commit d7f5013e12
1 changed files with 9 additions and 8 deletions
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17
hw/ppc/spapr_events.c
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@ -911,16 +911,17 @@ void spapr_mce_req_event(PowerPCCPU *cpu, bool recovered)
}
}
/*
* Try to block migration while FWNMI is being handled, so the
* machine check handler runs where the information passed to it
* actually makes sense. This shouldn't actually block migration,
* only delay it slightly, assuming migration is retried. If the
* attempt to block fails, carry on. Unfortunately, it always
* fails when running with -only-migrate. A proper interface to
* delay migration completion for a bit could avoid that.
*/
ret = migrate_add_blocker(spapr->fwnmi_migration_blocker, NULL);
if (ret == -EBUSY) {
/*
* We don't want to abort so we let the migration to continue.
* In a rare case, the machine check handler will run on the target.
* Though this is not preferable, it is better than aborting
* the migration or killing the VM. It is okay to call
* migrate_del_blocker on a blocker that was not added (which the
* nmi-interlock handler would do when it's called after this).
*/
warn_report("Received a fwnmi while migration was in progress");
}