Re: [patch 20/24] x86/speculation: Split out TIF update

[Tim Chen]

On 11/27/2018 02:36 PM, Thomas Gleixner wrote:

>>
>> We need this special handling only if the next task has TIF_SPEC_UPDATE 
>> set, which is one-off event globally (when seccomp marks all its threads 
>> so due to seccomp filter change), and once all the TIF_SPEC_UPDATE tasks 
>> schedule at least once, we're in a consistent state again and don't need 
>> this, as every running task will then have its TIF consistent with MSR 
>> value.
> 
> And how so? You set the bits is spec_flags. And then you set the TIF_UPDATE
> bit which is evaluated once.
> 
> Then you OR the bits into tifp which is a local variable and has nothing to
> do with the TIF flags of the next task. So on the next context switch this
> will evaluate the previous state of the TIF bits and you could have spared
> the whole exercise :)
> 

This is better than my original implementation which was racy.
Using task_spec_ssb_disable and task_spec_ib_disable to update TIF_* flags
at context switch time makes the update logic very clear
and extensible.

Thanks.

Tim