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

On 2/5/2026 11:25 AM, Greg KH wrote:

On Thu, Feb 05, 2026 at 05:10:30AM +0000, Nikunj A Dadhania wrote:

quoted

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@vger.kernel.org # 6.9+
Signed-off-by: Nikunj A Dadhania <redacted>
---

Reason to add stable tag:

With FRED support for SVM here 
https://lore.kernel.org/kvm/20260129063653.3553076-1-shivansh.dhiman@amd.com (local),
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