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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 > > We do need to include the complete description here as this is the final > patch of the series which fixes the kdump boot. > > Do note, that the description in the first patch only mentions the > IOMMU buffers - command, CWB and event buffers for reuse and this commit > log covers all reusing and remapping required - IOMMU buffers, device table, > etc. > >>> 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? >> > > As mentioned in the cover letter, this is the final patch of the series which > actually fixes the SNP kdump boot, so i kept Fixes: tag as part of this patch. > > I am not sure if i can add Fixes: tag to all the four patches in this series ? But just adding Fixes to this one patch is adding more confusion and complicating backport process. Is this really a fix? Did kdump ever worked on SNP enabled system? If yes then add Fixes to all patches. If not call it as an enhancement. -Vasant