nvme reservation commands during controller reset

[Christoph Hellwig]

On Fri, Aug 14, 2020 at 09:02:20AM -0700, Keith Busch wrote:
> Couldn't you just clear the REQ_DRIVER_FAILFAST flag to get the existing
> retry mechanisms working in this path?

That would only retry locally and thus not really help.

> 
> And while it looks fine for fabrics, PCIe doesn't have the implied HOST
> ID connection that makes reservations do the right thing across
> different paths. Automatic path failover retries for PCIe may have the
> wrong result.

You need a host id for reservations to work properly, as without that
they won't actually be persistent.  Unfortunasly NVMe allows
reservations without a hostid with some rather broken semantics instead
of just failing them, so applications that do use reservations needs
to ensure a host id is set.