[PATCH v2] x86/bugs: Warn when "ibrs" mitigation is selected on Enhanced IBRS parts

[Pawan Gupta <pawan.kumar.gupta@linux.intel.com>]

IBRS mitigation for spectre_v2 forces write to MSR_IA32_SPEC_CTRL at
every kernel entry/exit. On Enhanced IBRS parts setting
MSR_IA32_SPEC_CTRL[IBRS] only once at boot is sufficient. MSR writes at
every kernel entry/exit incur unnecessary performance loss.

When Enhanced IBRS feature is present, print a warning about this
unnecessary performance loss.

Signed-off-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>
---
v1->v2: Instead of changing the mitigation, print a warning about the
        perf loss.

v1: https://lore.kernel.org/lkml/0456b35fb9ef957d9a9138e0913fb1a3fd445dff.1657747493.git.pawan.kumar.gupta@linux.intel.com/

 arch/x86/kernel/cpu/bugs.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 0dd04713434b..1c54fad3c54b 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -975,6 +975,7 @@ 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_IBRS_PERF_MSG "WARNING: IBRS mitigation selected on Enhanced IBRS CPU, this may cause unnecessary performance loss\n"
 
 #ifdef CONFIG_BPF_SYSCALL
 void unpriv_ebpf_notify(int new_state)
@@ -1415,6 +1416,8 @@ static void __init spectre_v2_select_mitigation(void)
 
 	case SPECTRE_V2_IBRS:
 		setup_force_cpu_cap(X86_FEATURE_KERNEL_IBRS);
+		if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED))
+			pr_warn(SPECTRE_V2_IBRS_PERF_MSG);
 		break;
 
 	case SPECTRE_V2_LFENCE:

base-commit: 4a57a8400075bc5287c5c877702c68aeae2a033d
-- 
2.35.3