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

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Message-ID: <20260205051030.1225975-1-nikunj@amd.com>
Date: Thu, 5 Feb 2026 05:10:30 +0000
From: Nikunj A Dadhania <nikunj@....com>
To: <linux-kernel@...r.kernel.org>, <kvm@...r.kernel.org>, <bp@...en8.de>,
	<thomas.lendacky@....com>
CC: <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>, <nikunj@....com>,
	<stable@...r.kernel.org>
Subject: [PATCH] x86/fred: Fix early boot failures on SEV-ES/SNP guests

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.

---

 arch/x86/coco/sev/noinstr.c |  6 ++++++
 arch/x86/entry/entry_fred.c |  5 +++++
 arch/x86/kernel/fred.c      | 14 +++++++++++---
 3 files changed, 22 insertions(+), 3 deletions(-)

diff --git a/arch/x86/coco/sev/noinstr.c b/arch/x86/coco/sev/noinstr.c
index 9d94aca4a698..5afd663a1c21 100644
--- a/arch/x86/coco/sev/noinstr.c
+++ b/arch/x86/coco/sev/noinstr.c
@@ -121,6 +121,9 @@ noinstr struct ghcb *__sev_get_ghcb(struct ghcb_state *state)
 
 	WARN_ON(!irqs_disabled());
 
+	if (!sev_cfg.ghcbs_initialized)
+		return boot_ghcb;
+
 	data = this_cpu_read(runtime_data);
 	ghcb = &data->ghcb_page;
 
@@ -164,6 +167,9 @@ noinstr void __sev_put_ghcb(struct ghcb_state *state)
 
 	WARN_ON(!irqs_disabled());
 
+	if (!sev_cfg.ghcbs_initialized)
+		return;
+
 	data = this_cpu_read(runtime_data);
 	ghcb = &data->ghcb_page;
 
diff --git a/arch/x86/entry/entry_fred.c b/arch/x86/entry/entry_fred.c
index a9b72997103d..7a8659f19441 100644
--- a/arch/x86/entry/entry_fred.c
+++ b/arch/x86/entry/entry_fred.c
@@ -208,6 +208,11 @@ static noinstr void fred_hwexc(struct pt_regs *regs, unsigned long error_code)
 #ifdef CONFIG_X86_CET
 	case X86_TRAP_CP: return exc_control_protection(regs, error_code);
 #endif
+	case X86_TRAP_VC:
+		if (user_mode(regs))
+			return user_exc_vmm_communication(regs, error_code);
+		else
+			return kernel_exc_vmm_communication(regs, error_code);
 	default: return fred_bad_type(regs, error_code);
 	}
 
diff --git a/arch/x86/kernel/fred.c b/arch/x86/kernel/fred.c
index e736b19e18de..8cf4da546a8e 100644
--- a/arch/x86/kernel/fred.c
+++ b/arch/x86/kernel/fred.c
@@ -27,9 +27,6 @@ EXPORT_PER_CPU_SYMBOL(fred_rsp0);
 
 void cpu_init_fred_exceptions(void)
 {
-	/* When FRED is enabled by default, remove this log message */
-	pr_info("Initialize FRED on CPU%d\n", smp_processor_id());
-
 	/*
 	 * If a kernel event is delivered before a CPU goes to user level for
 	 * the first time, its SS is NULL thus NULL is pushed into the SS field
@@ -70,6 +67,17 @@ void cpu_init_fred_exceptions(void)
 	/* Use int $0x80 for 32-bit system calls in FRED mode */
 	setup_clear_cpu_cap(X86_FEATURE_SYSFAST32);
 	setup_clear_cpu_cap(X86_FEATURE_SYSCALL32);
+
+	/*
+	 * For secondary processors, FRED bit in CR4 gets enabled in cr4_init()
+	 * and FRED MSRs are not configured till the end of this function. For
+	 * SEV-ES and SNP guests, any console write before the FRED MSRs are
+	 * setup will cause a #VC and cannot be handled. Move the pr_info to
+	 * the end of this function.
+	 *
+	 * When FRED is enabled by default, remove this log message
+	 */
+	pr_info("Initialized FRED on CPU%d\n", smp_processor_id());
 }
 
 /* Must be called after setup_cpu_entry_areas() */

base-commit: 3c2ca964f75460093a8aad6b314a6cd558e80e66
-- 
2.48.1