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

[Haren Myneni]

On Thu, 2022-09-22 at 07:14 -0500, Nathan Lynch wrote:
> 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?

The user space resend the request to NX whenever the request is
returned with NX fault. So the process should be same even for faults
caused by the pre-migration.

Whereas the paste will be returned with failure when the window is
closed (unmap the paste address) and it can be considered as NX busy.
Up to the user space whether to send the request again after some delay
or fall back to SW compression and send the request again later.

For the migration, pre-migration event is notified to the hypervisor
and then OS will receive the migration event (SUSPEND) - So this patch
close windows early before VASI so that removing NX fault handling
during the time taken for VASI state transistion. 

Thanks
Haren