[PATCH v4 03/10] cpu/bugs: Fall back to AutoIBRS when retpoline unavailable on SNP CPUs

When the kernel is compiled without CONFIG_MITIGATION_RETPOLINE,
spectre_v2_select_retpoline() returns SPECTRE_V2_NONE, leaving SNP
hosts with AutoIBRS completely unmitigated against Spectre v2 in the
default/auto case.

Since SNP CPUs have AutoIBRS available, fall back to SPECTRE_V2_EIBRS
rather than leaving the system unmitigated.  The preceding commit already
enables AutoIBRS (and sets X86_FEATURE_IBRS_ENHANCED) on SNP parts, so no
additional feature-bit fixup is needed here; only the mitigation
selection changes when retpoline is unavailable.

Fixes: acaa4b5c4c85 ("x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled")
Cc: [email protected]
Signed-off-by: Kim Phillips <[email protected]>
Assisted-by: ClaudeCode:claude-opus-4-7
---
 arch/x86/kernel/cpu/bugs.c | 9 +++++++++
 1 file changed, 9 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index a813a98ac5bd..6a036b509f0b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -2204,6 +2204,15 @@ static void __init spectre_v2_select_mitigation(void)
 		}
 
 		spectre_v2_enabled = spectre_v2_select_retpoline();
+		/*
+		 * If retpoline is unavailable (e.g. built without
+		 * CONFIG_MITIGATION_RETPOLINE), fall back to eIBRS on
+		 * AutoIBRS-capable parts rather than leaving SNP hosts
+		 * unmitigated.
+		 */
+		if (spectre_v2_enabled == SPECTRE_V2_NONE &&
+		    boot_cpu_has(X86_FEATURE_AUTOIBRS))
+			spectre_v2_enabled = SPECTRE_V2_EIBRS;
 		break;
 
 	case SPECTRE_V2_CMD_RETPOLINE_LFENCE:
-- 
2.43.0