AMD/IOMMU: Cease using a dynamic height for the IOMMU pagetables

AMD/IOMMU: Cease using a dynamic height for the IOMMU pagetables

update_paging_mode() has multiple bugs:

 1) Booting with iommu=debug will cause it to inform you that that it called
    without the pdev_list lock held.
 2) When growing by more than a single level, it leaks the newly allocated
    table(s) in the case of a further error.

Furthermore, the choice of default level for a domain has issues:

 1) All HVM guests grow from 2 to 3 levels during construction because of the
    position of the VRAM just below the 4G boundary, so defaulting to 2 is a
    waste of effort.
 2) The limit for PV guests doesn't take memory hotplug into account, and
    isn't dynamic at runtime like HVM guests.  This means that a PV guest may
    get RAM which it can't map in the IOMMU.

The dynamic height is a property unique to AMD, and adds a substantial
quantity of complexity for what is a marginal performance improvement.  Remove
the complexity by removing the dynamic height.

PV guests now get 3 or 4 levels based on any hotplug regions in the host.
This only makes a difference for hardware which previously had all RAM below
the 512G boundary, and a hotplug region above.

HVM guests now get 4 levels (which will be sufficient until 256TB guests
become a thing), because we don't currently have the information to know when
3 would be safe to use.

The overhead of this extra level is not expected to be noticeable.  It costs
one page (4k) per domain, and one extra IO-TLB paging structure cache entry
which is very hot and less likely to be evicted.

This is XSA-311.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>
master commit: b4f042236ae0bb6725b3e8dd40af5a2466a6f971
master date: 2019-12-11 14:55:32 +0100