Re: [PATCH v10 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID

[H. Peter Anvin]

On 03/14/17 12:01, H. Peter Anvin wrote:
> On 11/08/16 10:39, Kyle Huey wrote:
>>  	}
>>  
>> +	if (test_tsk_thread_flag(prev_p, TIF_NOCPUID) ^
>> +	    test_tsk_thread_flag(next_p, TIF_NOCPUID)) {
>> +		set_cpuid_faulting(test_tsk_thread_flag(next_p, TIF_NOCPUID));
>> +	}
>> +
>>  	if (test_tsk_thread_flag(prev_p, TIF_NOTSC) ^
>>  	    test_tsk_thread_flag(next_p, TIF_NOTSC)) {
>>  		/* prev and next are different */
>>  		if (test_tsk_thread_flag(next_p, TIF_NOTSC))
>>  			hard_disable_TSC();
>>  		else
>>  			hard_enable_TSC();
>>  	}
> 
> I'm unhappy about this part: we already do two XORs on these after bit
> extraction, which is quite inefficient; and at least theoretically we
> could be indirecting though the ->stack pointer for every one if gcc
> can't tell it won't have changed (we really need to get thread_info
> moved into the task_struct allocation and away from the kernel stack,
> especially since on x86 the pointer is the same size as the vestigial
> structure it points to.)
> 

Nevermind, I was accidentally looking at v10 not v15 of this patchset.
My bad.

	-hpa