iommu/amd: Add support to remap/unmap IOMMU buffers for kdump

iommu/amd: Add support to remap/unmap IOMMU buffers for kdump

After a panic if SNP is enabled in the previous kernel then the kdump
kernel boots with IOMMU SNP enforcement still enabled.

IOMMU completion wait buffers (CWBs), command buffers and event buffer
registers remain locked and exclusive to the previous kernel. Attempts
to allocate and use new buffers in the kdump kernel fail, as hardware
ignores writes to the locked MMIO registers as per AMD IOMMU spec
Section 2.12.2.1.

This results in repeated "Completion-Wait loop timed out" errors and a
second kernel panic: "Kernel panic - not syncing: timer doesn't work
through Interrupt-remapped IO-APIC"

The list of MMIO registers locked and which ignore writes after failed
SNP shutdown are mentioned in the AMD IOMMU specifications below:

Section 2.12.2.1.
https://docs.amd.com/v/u/en-US/48882_3.10_PUB

Reuse the pages of the previous kernel for completion wait buffers,
command buffers, event buffers and memremap them during kdump boot
and essentially work with an already enabled IOMMU configuration and
re-using the previous kernel’s data structures.

Reusing of command buffers and event buffers is now done for kdump boot
irrespective of SNP being enabled during kdump.

Re-use of completion wait buffers is only done when SNP is enabled as
the exclusion base register is used for the completion wait buffer
(CWB) address only when SNP is enabled.

Reviewed-by: Vasant Hegde <vasant.hegde@amd.com>
Tested-by: Sairaj Kodilkar <sarunkod@amd.com>
Signed-off-by: Ashish Kalra <ashish.kalra@amd.com>
Link: https://lore.kernel.org/r/ff04b381a8fe774b175c23c1a336b28bc1396511.1756157913.git.ashish.kalra@amd.com
Signed-off-by: Joerg Roedel <joerg.roedel@amd.com>