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

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Message-ID: <aYS8MQLcGs08PxYK@google.com>
Date: Thu, 5 Feb 2026 07:50:09 -0800
From: Sean Christopherson <seanjc@...gle.com>
To: Greg KH <gregkh@...uxfoundation.org>
Cc: "Nikunj A. Dadhania" <nikunj@....com>, 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, 
	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, Greg KH wrote:
> On Thu, Feb 05, 2026 at 11:40:11AM +0530, Nikunj A. Dadhania wrote:
> > 
> > 
> > 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@...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 :)

I disagree, this absolutely is a regression.  Kernels without commit 14619d912b65
will boot on this "new" hardware, kernels with the commit will not.

> > 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.
> 
> Great, then upgrade those guest kernels as they have never been able to
> run on those hypervisors :)

As above, *upgrading* from e.g. 6.6 to 6.12 will suddenly fail to boot.

> > Other option would be to disable FRED for SEV-ES and SNP guest in stable kernel.
> 
> That's a choice for the hypervisor vendors to choose.

No, because the hypervisor has no clue what kernel version the guest is running.