x86/bugs: Warn when "ibrs" mitigation is selected on Enhanced IBRS parts

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>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Cc: stable@vger.kernel.org
Link: https://lore.kernel.org/r/2a5eaf54583c2bfe0edc4fea64006656256cca17.1657814857.git.pawan.kumar.gupta@linux.intel.com

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: