David Gibson <da...@gibson.dropbear.id.au> writes: > On Thu, Jul 15, 2021 at 03:32:06PM +0200, Markus Armbruster wrote: >> Commit 2500fb423a "migration: Include migration support for machine >> check handling" adds this: >> >> ret = migrate_add_blocker(spapr->fwnmi_migration_blocker, &local_err); >> 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. >> */ >> warn_report("Received a fwnmi while migration was in progress"); >> } >> >> migrate_add_blocker() can fail in two ways: >> >> 1. -EBUSY: migration is already in progress >> >> Ignoring this one is clearly intentional. The comment explains why. >> I'm taking it at face value (I'm a spapr ignoramus). > > Right. The argument isn't really about papr particularly, except > insofar as understanding what fwnmi is. fwnmi (FirmWare assisted NMI) > is a reporting mechanism for certain low-level hardware failures > (think memory ECC or cpu level faults, IIRC). If we migrate between > detecting and reporting the error, then the particulars we report will > be mostly meaningless since they relate to hardware we're no longer > running on. Hence the migration blocker. > > However, migrating away from a (non-fatal) fwnmi error is a pretty > reasonable response, so we don't want to actually fail a migration if > its already in progress. > >> Aside: I doubt >> the warning is going to help users. > > You're probably right, but it's not very clear how to do better. It > might possibly help someone in tech support explain why the reported > fwnmi doesn't seem to match the hardware the guest is (now) running > on.
Perhaps pointing to the actual problem could help: the FWNMI's information is mostly meaningless. >> 2. -EACCES: we're running with -only-migratable >> >> Why may we ignore -only-migratable here? > > Short answer: because I didn't think about that case. Long answer: > I think we probably shoud ignore it anyway. As above, receiving a > fwnmi doesn't really prevent migration, it just means that if you're > unlucky it can report stale information. Since migrating away from a > possibly-dubious host would be a reasonable response to a non-fatal > fwnmi, I don't think we want to simply prohibit fwnmi entirely with > -only-migratable. I think the comment text and placement could be improved to make clear ignoring this failure is intentional, too. How do you like the following? diff --git a/hw/ppc/spapr_events.c b/hw/ppc/spapr_events.c index a8f2cc6bdc..54d8e856d3 100644 --- a/hw/ppc/spapr_events.c +++ b/hw/ppc/spapr_events.c @@ -911,16 +911,14 @@ 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 won't actually block migration, + * only delay it slightly. If the attempt fails, carry on. + */ 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"); } >> By the way, we leak @local_err on failure. I'll post a patch, but I'd >> like my question answered first.