[PATCH v3 1/6] cpu/bugs: Allow forcing Automatic IBRS with SNP active using spectre_v2=eibrs

[Kim Phillips <kim.phillips@amd.com>]

spectre_v2=eibrs currently enables retpolines when SNP is enabled,
instead of AutoIBRS (EIBRS) because the commit that disabled
AutoIBRS if SNP is enabled stopped short of enabling
X86_FEATURE_IBRS_ENHANCED.

Change the logic to enable X86_FEATURE_IBRS_ENHANCED, and move the
decision to switch to retpolines in the default/"auto" case in
spectre_v2_select_mitigation().  This allows the existing
spectre_v2=eibrs logic to work as intended.

Also emit a performance loss warning for using AutoIBRS with
SNP enabled.

Fixes: acaa4b5c4c85 ("x86/speculation: Do not enable Automatic IBRS if SEV-SNP is enabled")
Reported-by: Tom Lendacky <thomas.lendacky@amd.com>
Cc: Borislav Petkov (AMD) <bp@alien8.de>
Cc: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Sean Christopherson <seanjc@google.com>
Cc: stable@kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202603121136.bc8zNsHS-lkp@intel.com/
Signed-off-by: Kim Phillips <kim.phillips@amd.com>
---
v3:
 - Addressed Pawan Gupta's comment and remove wrong SPECTRE_V2_CMD_FORCE ("=on") check
 - Addressed kernel test robot's !A || A && B is equivalent to !A || B warning
 - Preferred to add new AutoIBRS with SEV-SNP enabled performance warning instead
   of muting legacy IBRS in use vs. eIBRS messaging in the context of SNP, since
   SNP users' IBRS performance varies whether they enable SNP BTB Isolation

v2: https://lore.kernel.org/kvm/20260311130611.2201214-2-kim.phillips@amd.com/
 - Address Dave Hansen's comment to adhere to using the IBRS_ENHANCED
   Intel feature flag also for AutoIBRS.

v1:
 https://lore.kernel.org/kvm/20260224180157.725159-2-kim.phillips@amd.com/

 arch/x86/kernel/cpu/bugs.c   | 10 +++++++++-
 arch/x86/kernel/cpu/common.c |  6 +-----
 2 files changed, 10 insertions(+), 6 deletions(-)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 83f51cab0b1e..dfefbde10646 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -1658,6 +1658,7 @@ static inline const char *spectre_v2_module_string(void) { return ""; }
 #define SPECTRE_V2_LFENCE_MSG "WARNING: LFENCE mitigation is not recommended for this CPU, data leaks possible!\n"
 #define SPECTRE_V2_EIBRS_EBPF_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS on, data leaks possible via Spectre v2 BHB attacks!\n"
 #define SPECTRE_V2_EIBRS_LFENCE_EBPF_SMT_MSG "WARNING: Unprivileged eBPF is enabled with eIBRS+LFENCE mitigation and SMT, data leaks possible via Spectre v2 BHB attacks!\n"
+#define SPECTRE_V2_EIBRS_SNP_PERF_MSG "WARNING: AutoIBRS mitigation selected on SEV-SNP enabled CPU, this may cause unnecessary performance loss\n"
 #define SPECTRE_V2_IBRS_PERF_MSG "WARNING: IBRS mitigation selected on Enhanced IBRS CPU, this may cause unnecessary performance loss\n"
 
 #ifdef CONFIG_BPF_SYSCALL
@@ -2181,7 +2182,12 @@ static void __init spectre_v2_select_mitigation(void)
 			break;
 		fallthrough;
 	case SPECTRE_V2_CMD_FORCE:
-		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) {
+		/*
+		 * Don't use AutoIBRS when SNP is enabled because it degrades
+		 * host userspace indirect branch performance.
+		 */
+		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED) &&
+		    !boot_cpu_has(X86_FEATURE_SEV_SNP)) {
 			spectre_v2_enabled = SPECTRE_V2_EIBRS;
 			break;
 		}
@@ -2257,6 +2263,8 @@ static void __init spectre_v2_apply_mitigation(void)
 		return;
 
 	case SPECTRE_V2_EIBRS:
+		if (boot_cpu_has(X86_FEATURE_SEV_SNP))
+			pr_warn(SPECTRE_V2_EIBRS_SNP_PERF_MSG);
 		break;
 
 	case SPECTRE_V2_IBRS:
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 4e1f0c4afe3a..0cdcbbedf883 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -1485,13 +1485,9 @@ static void __init cpu_set_bug_bits(struct cpuinfo_x86 *c)
 	/*
 	 * AMD's AutoIBRS is equivalent to Intel's eIBRS - use the Intel feature
 	 * flag and protect from vendor-specific bugs via the whitelist.
-	 *
-	 * Don't use AutoIBRS when SNP is enabled because it degrades host
-	 * userspace indirect branch performance.
 	 */
 	if ((x86_arch_cap_msr & ARCH_CAP_IBRS_ALL) ||
-	    (cpu_has(c, X86_FEATURE_AUTOIBRS) &&
-	     !cpu_feature_enabled(X86_FEATURE_SEV_SNP))) {
+	    cpu_has(c, X86_FEATURE_AUTOIBRS)) {
 		setup_force_cpu_cap(X86_FEATURE_IBRS_ENHANCED);
 		if (!cpu_matches(cpu_vuln_whitelist, NO_EIBRS_PBRSB) &&
 		    !(x86_arch_cap_msr & ARCH_CAP_PBRSB_NO))
-- 
2.43.0