Re: [PATCH] x86/fred: Fix early boot failures on SEV-ES/SNP guests

[Nikunj A. Dadhania]

On 2/5/2026 11:25 AM, Greg KH wrote:
> On Thu, Feb 05, 2026 at 05:10:30AM +0000, Nikunj A Dadhania wrote:
>> FRED enabled SEV-ES and SNP guests fail to boot due to the following
>> issues in the early boot sequence:
>>
>> * FRED does not have a #VC exception handler in the dispatch logic
>>
>> * For secondary CPUs, FRED is enabled before setting up the FRED MSRs, and
>>   console output triggers a #VC which cannot be handled
>>
>> * Early FRED #VC exceptions should use boot_ghcb until per-CPU GHCBs are
>>   initialized
>>
>> Fix these issues to ensure SEV-ES/SNP guests can handle #VC exceptions
>> correctly during early boot when FRED is enabled.
>>
>> Fixes: 14619d912b65 ("x86/fred: FRED entry/exit and dispatch code")
>> Cc: stable@xxxxxxxxxxxxxxx # 6.9+
>> Signed-off-by: Nikunj A Dadhania <nikunj@xxxxxxx>
>> ---
>>
>> Reason to add stable tag:
>>
>> With FRED support for SVM here 
>> https://lore.kernel.org/kvm/20260129063653.3553076-1-shivansh.dhiman@xxxxxxx,
>> SVM and SEV guests running 6.9 and later kernels will support FRED.
>> However, *SEV-ES and SNP guests cannot support FRED* and will fail to boot
>> with the following error:
>>
>>     [    0.005144] Using GB pages for direct mapping
>>     [    0.008402] Initialize FRED on CPU0
>>     qemu-system-x86_64: cpus are not resettable, terminating
>>
>> Three problems were identified as detailed in the commit message above and
>> is fixed with this patch.
>>
>> I would like the patch to be backported to the LTS kernels (6.12 and 6.18) to
>> ensure SEV-ES and SNP guests running these stable kernel versions can boot
>> with FRED enabled on FRED-enabled hypervisors.
> 
> That sounds like new hardware support, if you really want that, why not
> just use newer kernel versions with this fix in it?  Obviously no one is
> running those kernels on that hardware today, so this isn't a regression :)

Fair point.

However, the situation is a bit nuanced: FRED hardware is available now, and
users running current stable kernels as guests will encounter boot
failures when the hypervisor is updated to support FRED. While not a traditional
regression, it creates a compatibility gap where stable guest kernels cannot run
on updated hypervisors.

Other option would be to disable FRED for SEV-ES and SNP guest in stable kernel.

Regards
Nikunj