Re: [Patch v3 03/13] x86/speculation: Add static key for Enhanced IBRS

[Tim Chen]

On 10/26/2018 09:58 AM, Waiman Long wrote:
> On 10/17/2018 01:59 PM, Tim Chen wrote:
>> Add static key to indicate whether we are using Enhanced IBRS to mitigate
>> Spectre v2.  This will be used in later patches to disengage STIBP code
>> for Spectre v2 mitigation as STIBP is not needed when Enhanced IBRS is
>> in use.
>>
>> Signed-off-by: Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx>
>> ---
>>  arch/x86/include/asm/nospec-branch.h | 3 +++
>>  arch/x86/kernel/cpu/bugs.c           | 4 ++++
>>  2 files changed, 7 insertions(+)
>>
>> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
>> index fd2a8c1..d57e84e 100644
>> --- a/arch/x86/include/asm/nospec-branch.h
>> +++ b/arch/x86/include/asm/nospec-branch.h
>> @@ -3,6 +3,7 @@
>>  #ifndef _ASM_X86_NOSPEC_BRANCH_H_
>>  #define _ASM_X86_NOSPEC_BRANCH_H_
>>  
>> +#include <linux/static_key.h>
>>  #include <asm/alternative.h>
>>  #include <asm/alternative-asm.h>
>>  #include <asm/cpufeatures.h>
>> @@ -228,6 +229,8 @@ enum ssb_mitigation {
>>  extern char __indirect_thunk_start[];
>>  extern char __indirect_thunk_end[];
>>  
>> +DECLARE_STATIC_KEY_FALSE(spectre_v2_enhanced_ibrs);
>> +
>>  /*
>>   * On VMEXIT we must ensure that no RSB predictions learned in the guest
>>   * can be followed in the host, by overwriting the RSB completely. Both
>> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
>> index b2f6b8b..2fc7b4e 100644
>> --- a/arch/x86/kernel/cpu/bugs.c
>> +++ b/arch/x86/kernel/cpu/bugs.c
>> @@ -142,6 +142,9 @@ static const char *spectre_v2_strings[] = {
>>  	[SPECTRE_V2_IBRS_ENHANCED]		= "Mitigation: Enhanced IBRS",
>>  };
>>  
>> +DEFINE_STATIC_KEY_FALSE(spectre_v2_enhanced_ibrs);
>> +EXPORT_SYMBOL(spectre_v2_enhanced_ibrs);
>> +
>>  #undef pr_fmt
>>  #define pr_fmt(fmt)     "Spectre V2 : " fmt
>>  
>> @@ -386,6 +389,7 @@ static void __init spectre_v2_select_mitigation(void)
>>  			/* Force it so VMEXIT will restore correctly */
>>  			x86_spec_ctrl_base |= SPEC_CTRL_IBRS;
>>  			wrmsrl(MSR_IA32_SPEC_CTRL, x86_spec_ctrl_base);
>> +			static_branch_enable(&spectre_v2_enhanced_ibrs);
>>  			goto specv2_set_mode;
>>  		}
>>  		if (IS_ENABLED(CONFIG_RETPOLINE))
> 
> Why you need a static key for enhanced IBRS? It is supposed to be set at
> boot time and never get changed after that. It will be easier to use a
> feature bit for that instead. We usually use static key when the value
> can be changed at run time.
>

We're close to running out of the feature bits.  So I'm trying not to
use those.

Tim