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

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Date:   Tue, 27 Nov 2018 17:50:48 -0800
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Jiri Kosina <jikos@...nel.org>
Cc:     Ingo Molnar <mingo@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch 20/24] x86/speculation: Split out TIF update

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