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Ashish Kalra 653398d19a iommu/amd: Add support to remap/unmap IOMMU buffers for kdump 
[ Upstream commit f32fe7cb01 ]

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>
Signed-off-by: Sasha Levin <sashal@kernel.org>
History 2025-11-13 15:37:14 -05:00
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README 

Linux kernel
============

There are several guides for kernel developers and users. These guides can
be rendered in a number of formats, like HTML and PDF. Please read
Documentation/admin-guide/README.rst first.

In order to build the documentation, use ``make htmldocs`` or
``make pdfdocs``.  The formatted documentation can also be read online at:

    https://www.kernel.org/doc/html/latest/

There are various text files in the Documentation/ subdirectory,
several of them using the reStructuredText markup notation.

Please read the Documentation/process/changes.rst file, as it contains the
requirements for building and running the kernel, and information about
the problems which may result by upgrading your kernel.