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

[Liu Song <liusong@linux.alibaba.com>]

> On Fri, Aug 05, 2022 at 05:21:14PM +0800, Liu Song wrote:
>> From: Liu Song <liusong@linux.alibaba.com>
>>
>> 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@linux.alibaba.com>
>> ---
>>   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