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From: Kyle Huey <me@kylehuey.com>
Date: Tue, 14 Mar 2017 12:08:31 -0700
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Subject: Re: [PATCH v10 6/7] x86/arch_prctl: Add ARCH_[GET|SET]_CPUID
To: "H. Peter Anvin" <hpa@zytor.com>
Cc: "Robert O'Callahan" <robert@ocallahan.org>,
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On Tue, Mar 14, 2017 at 12:01 PM, H. Peter Anvin <hpa@zytor.com> 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.)
>
> It would be so much saner to do one xor and then go onto a common slow path:
>
>         struct thread_info *prev_ti = task_thread_info(prev_p);
>         struct thread_info *next_ti = task_thread_info(next_p);
>
>         tif_flipped = prev_ti->flags ^ next_ti->flags;
>
>         if (unlikely(tif_flipped &
>                 (_TIF_BLOCKSTEP | _TIF_NOTSC | _TIF_NOCPUID))) {
>                 if (tif_flipped & _TIF_BLOCKSTEP) {
>                         ...
>                 }
>                 if (tif_flipped & _TIF_NOTSC) {
>                         ...
>                 }
>                 if (tif_flipped & _TIF_NOCPUID) {
>                         ...
>                 }
>         }
>
> Then we can also replace test_tsk_thread_flag() with
> test_ti_thread_flag() in other places in this function.

That's largely what we ended up doing.  See
https://lkml.org/lkml/2017/2/14/80 and the latest version of this
patch, https://lkml.org/lkml/2017/3/11/197.

- Kyle