[PATCH] powerpc/pseries: Move vas_migration_handler early during migration

[Nathan Lynch]

Haren Myneni <haren at linux.ibm.com> writes:
> When the migration is initiated, the hypervisor changes VAS
> mappings as part of pre-migration event. Then the OS gets the
> migration event which closes all VAS windows before the migration
> starts. NX generates continuous faults until windows are closed
> and the user space can not differentiate these NX faults coming
> from the actual migration. So to reduce this time window, close
> VAS windows first in pseries_migrate_partition().

I'm concerned that this is only narrowing a window of time where
undesirable faults occur, and that it may not be sufficient for all
configurations. Migrations can be in progress for minutes or hours,
while the time that we wait for the VASI state transition is usually
seconds or minutes. So I worry that this works around a problem in
limited cases but doesn't cover them all.

Maybe I don't understand the problem well enough. How does user space
respond to the NX faults?