Re: [PATCH v4 1/4] iommu/amd: Add support to remap/unmap IOMMU buffers for kdump

[Sairaj Kodilkar]

On 7/22/2025 3:22 AM, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@xxxxxxx>
>
> 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.
>
> Signed-off-by: Ashish Kalra <ashish.kalra@xxxxxxx>
Tested-by: Sairaj Kodilkar <sarunkod@xxxxxxx>