iommu/amd: Workaround for erratum 787

iommu/amd: Workaround for erratum 787

The IOMMU interrupt handling in bottom half must clear the PPR log interrupt
and event log interrupt bits to re-enable the interrupt. This is done by
writing 1 to the memory mapped register to clear the bit. Due to hardware bug,
if the driver tries to clear this bit while the IOMMU hardware also setting
this bit, the conflict will result with the bit being set. If the interrupt
handling code does not make sure to clear this bit, subsequent changes in the
event/PPR logs will no longer generating interrupts, and would result if
buffer overflow. After clearing the bits, the driver must read back
the register to verify.

Signed-off-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>
Adjust to apply on top of heavily modified patch 1. Adjust flow to get away
with a single readl() in each instance of the status register checks.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Acked-by: Suravee Suthikulpanit <suravee.suthikulpanit@amd.com>