On Thu, Jul 14, 2022 at 04:15:35PM -0700, Pawan Gupta wrote: > 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@xxxxxxxxxxxxxxx> Reviewed-by: Thadeu Lima de Souza Cascardo <cascardo@xxxxxxxxxxxxx> > --- > 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@xxxxxxxxxxxxxxx/ > > 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 > >
