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

["Kalra, Ashish" <ashish.kalra@amd.com>]

Hello Vasant,

On 7/17/2025 1:22 AM, Vasant Hegde wrote:
> 
> 
> On 7/17/2025 3:42 AM, Kalra, Ashish wrote:
>> Hello Vasant,
>>
>> On 7/16/2025 4:46 AM, Vasant Hegde wrote:
>>>
>>>
>>> On 7/16/2025 12:57 AM, Ashish Kalra wrote:
>>>> From: Ashish Kalra <ashish.kalra@amd.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
>>
>> 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.
> 

Well, kdump only worked on SNP enabled systems if there are no active SNP VMs.

But i think it makes more sense to remove the Fixes: tag from these patch-series
as this SNP kdump support is more or less a feature enhancement for SNP.

Thanks,
Ashish