linux-kernel - Re: [PATCH v3 4/4] iommu/amd: Fix host kdump support for SNP

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Message-ID: <529c8436-1aeb-41bc-94bd-8b0f128e6222@amd.com>
Date: Wed, 16 Jul 2025 15:16:27 +0530
From: Vasant Hegde <vasant.hegde@....com>
To: Ashish Kalra <Ashish.Kalra@....com>, joro@...tes.org,
 suravee.suthikulpanit@....com, thomas.lendacky@....com,
 Sairaj.ArunKodilkar@....com, herbert@...dor.apana.org.au
Cc: seanjc@...gle.com, pbonzini@...hat.com, will@...nel.org,
 robin.murphy@....com, john.allen@....com, davem@...emloft.net, bp@...en8.de,
 michael.roth@....com, iommu@...ts.linux.dev, linux-kernel@...r.kernel.org,
 linux-crypto@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [PATCH v3 4/4] iommu/amd: Fix host kdump support for SNP



On 7/16/2025 12:57 AM, Ashish Kalra wrote:
> From: Ashish Kalra <ashish.kalra@....com>
> 
> When a crash is triggered the kernel attempts to shut down SEV-SNP
> using the SNP_SHUTDOWN_EX command. If active SNP VMs are present,
> SNP_SHUTDOWN_EX fails as firmware checks all encryption-capable ASIDs
> to ensure none are in use and that a DF_FLUSH is not required. If a
> DF_FLUSH is required, the firmware returns DFFLUSH_REQUIRED, causing
> SNP_SHUTDOWN_EX to fail.
> 
> This casues the kdump kernel to boot with IOMMU SNP enforcement still
> enabled and IOMMU completion wait buffers (CWBs), command buffers,
> device tables 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 the hardware ignores writes to the locked
> MMIO registers (per AMD IOMMU spec Section 2.12.2.1).
> 
> As a result, the kdump kernel cannot initialize the IOMMU or enable IRQ
> remapping which is required for proper operation.
> 
> 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 following MMIO registers are locked and ignore writes after failed
> SNP shutdown:
> Device Table Base Address Register
> Command Buffer Base Address Register
> Event Buffer Base Address Register
> Completion Store Base Register/Exclusion Base Register
> Completion Store Limit Register/Exclusion Range Limit Register
> 

May be you can rephrase the description as first patch covered some of these
details.

> Instead of allocating new buffers, re-use the previous kernel’s pages
> for completion wait buffers, command buffers, event buffers and device
> tables and operate with the already enabled SNP configuration and
> existing data structures.
> 
> This approach is now used for kdump boot regardless of whether SNP is
> enabled during kdump.
> 
> The fix enables successful crashkernel/kdump operation on SNP hosts
> even when SNP_SHUTDOWN_EX fails.
> 
> Fixes: c3b86e61b756 ("x86/cpufeatures: Enable/unmask SEV-SNP CPU feature")

I am not sure why you have marked only this patch as Fixes? Also it won't fix
the kdump if someone just backports only this patch right?

-Vasant