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

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Message-ID: <2026020559-igloo-revolver-1442@gregkh>
Date: Thu, 5 Feb 2026 06:55:40 +0100
From: Greg KH <gregkh@...uxfoundation.org>
To: Nikunj A Dadhania <nikunj@....com>
Cc: linux-kernel@...r.kernel.org, kvm@...r.kernel.org, bp@...en8.de,
	thomas.lendacky@....com, tglx@...nel.org, mingo@...hat.com,
	dave.hansen@...ux.intel.com, hpa@...or.com, xin@...or.com,
	seanjc@...gle.com, pbonzini@...hat.com, x86@...nel.org,
	jon.grimm@....com, stable@...r.kernel.org
Subject: Re: [PATCH] x86/fred: Fix early boot failures on SEV-ES/SNP guests

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@...r.kernel.org # 6.9+
> Signed-off-by: Nikunj A Dadhania <nikunj@....com>
> ---
> 
> Reason to add stable tag:
> 
> With FRED support for SVM here 
> https://lore.kernel.org/kvm/20260129063653.3553076-1-shivansh.dhiman@amd.com,
> 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 :)

thanks,

greg k-h