Re: [PATCH] arm64: spectre: increase parameters that can be used to turn off bhb mitigation individually

[Liu Song]

> On Fri, Aug 05, 2022 at 05:21:14PM +0800, Liu Song wrote:
> > From: Liu Song <liusong@xxxxxxxxxxxxxxxxx>
> >
> > In our environment, it was found that the mitigation BHB has a great
> > impact on the benchmark performance. For example, in the lmbench test,
> > the "process fork && exit" test performance drops by 20%.
> > So it is necessary to have the ability to turn off the mitigation
> > individually through cmdline, thus avoiding having to compile the
> > kernel by adjusting the config.
> >
> > Signed-off-by: Liu Song <liusong@xxxxxxxxxxxxxxxxx>
> > ---
> >   arch/arm64/kernel/proton-pack.c | 10 +++++++++-
> >   1 file changed, 9 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/arm64/kernel/proton-pack.c b/arch/arm64/kernel/proton-pack.c
> > index 40be3a7..bd16903 100644
> > --- a/arch/arm64/kernel/proton-pack.c
> > +++ b/arch/arm64/kernel/proton-pack.c
> > @@ -988,6 +988,14 @@ static void this_cpu_set_vectors(enum arm64_bp_harden_el1_vectors slot)
> >   	isb();
> >   }
> >   
> > +static bool __read_mostly __nospectre_bhb;
> > +static int __init parse_spectre_bhb_param(char *str)
> > +{
> > +	__nospectre_bhb = true;
> > +	return 0;
> > +}
> > +early_param("nospectre_bhb", parse_spectre_bhb_param);
> > +
> >   void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
> >   {
> >   	bp_hardening_cb_t cpu_cb;
> > @@ -1001,7 +1009,7 @@ void spectre_bhb_enable_mitigation(const struct arm64_cpu_capabilities *entry)
> >   		/* No point mitigating Spectre-BHB alone. */
> >   	} else if (!IS_ENABLED(CONFIG_MITIGATE_SPECTRE_BRANCH_HISTORY)) {
> >   		pr_info_once("spectre-bhb mitigation disabled by compile time option\n");
> > -	} else if (cpu_mitigations_off()) {
> > +	} else if (cpu_mitigations_off() || __nospectre_bhb) {
> >   		pr_info_once("spectre-bhb mitigation disabled by command line option\n");
> >   	} else if (supports_ecbhb(SCOPE_LOCAL_CPU)) {
> >   		state = SPECTRE_MITIGATED;
> It would be good to have an Ack (or a shrug) from somebody @arm.com on this
> one.
>
> Other than that, the documentation needs updating for the new option.

Thanks for the reminder, I will post another V2 version.


Thanks


> Will