From: Jiri Kosina <jkos...@suse.cz> STIBP is a feature provided by certain Intel ucodes / CPUs. This feature (once enabled) prevents cross-hyperthread control of decisions made by indirect branch predictors.
Enable this feature if - the CPU is vulnerable to spectre v2 - the CPU supports SMT - spectre_v2 mitigation autoselection is enabled (default) After some previous discussion, this patch leaves STIBP on all the time, as wrmsr on crossing kernel boundary is a no-no. This could perhaps later be a bit more optimized (like disabling it in NOHZ, experiment with disabling it in idle, etc) if needed. Cc: sta...@vger.kernel.org Signed-off-by: Jiri Kosina <jkos...@suse.cz> --- arch/x86/kernel/cpu/bugs.c | 12 ++++++++++++ 1 file changed, 12 insertions(+) diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c index 40bdaea97fe7..535b006dc50d 100644 --- a/arch/x86/kernel/cpu/bugs.c +++ b/arch/x86/kernel/cpu/bugs.c @@ -325,6 +325,12 @@ static enum spectre_v2_mitigation_cmd __init spectre_v2_parse_cmdline(void) return cmd; } +static bool __init stibp_needed(void) +{ + return (cpu_smt_control != CPU_SMT_NOT_SUPPORTED && + boot_cpu_has(X86_FEATURE_STIBP)); +} + static void __init spectre_v2_select_mitigation(void) { enum spectre_v2_mitigation_cmd cmd = spectre_v2_parse_cmdline(); @@ -344,6 +350,12 @@ static void __init spectre_v2_select_mitigation(void) case SPECTRE_V2_CMD_FORCE: case SPECTRE_V2_CMD_AUTO: + if (stibp_needed()) { + /* Enable STIBP on SMT-capable systems */ + pr_info("Spectre v2 cross-process SMT mitigation: Enabling STIBP\n"); + x86_spec_ctrl_base |= SPEC_CTRL_STIBP; + wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base); + } if (boot_cpu_has(X86_FEATURE_IBRS_ENHANCED)) { mode = SPECTRE_V2_IBRS_ENHANCED; /* Force it so VMEXIT will restore correctly */ -- Jiri Kosina SUSE Labs