iommu/amd: Recover from event log overflow

The AMD IOMMU logs I/O page faults and such to a ring buffer in
system memory, and this ring buffer can overflow.  The AMD IOMMU
spec has the following to say about the interrupt status bit that
signals this overflow condition:

	EventOverflow: Event log overflow. RW1C. Reset 0b. 1 = IOMMU
	event log overflow has occurred. This bit is set when a new
	event is to be written to the event log and there is no usable
	entry in the event log, causing the new event information to
	be discarded. An interrupt is generated when EventOverflow = 1b
	and MMIO Offset 0018h[EventIntEn] = 1b. No new event log
	entries are written while this bit is set. Software Note: To
	resume logging, clear EventOverflow (W1C), and write a 1 to
	MMIO Offset 0018h[EventLogEn].

The AMD IOMMU driver doesn't currently implement this recovery
sequence, meaning that if a ring buffer overflow occurs, logging
of EVT/PPR/GA events will cease entirely.

This patch implements the spec-mandated reset sequence, with the
minor tweak that the hardware seems to want to have a 0 written to
MMIO Offset 0018h[EventLogEn] first, before writing an 1 into this
field, or the IOMMU won't actually resume logging events.

Signed-off-by: Lennert Buytenhek <buytenh@arista.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/YVrSXEdW2rzEfOvk@wantstofly.org


Signed-off-by: Joerg Roedel <jroedel@suse.de>