linux-kernel - Re: [PATCH v4 1/7] x86/fsgsbase/64: Introduce FS/GS base helper functions

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Message-Id: <851713FF-90E1-434A-BC31-5DE437AB530E@amacapital.net>
Date:   Fri, 22 Jun 2018 09:08:59 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Andy Lutomirski <luto@...nel.org>,
        "Bae, Chang Seok" <chang.seok.bae@...el.com>,
        "H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v4 1/7] x86/fsgsbase/64: Introduce FS/GS base helper functions



> On Jun 22, 2018, at 8:39 AM, Thomas Gleixner <tglx@...utronix.de> wrote:
> 
>> On Fri, 22 Jun 2018, Andy Lutomirski wrote:
>> On Fri, Jun 22, 2018 at 7:28 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>>> +unsigned long read_task_fsbase(struct task_struct *task)
>>>> +{
>>>> +     unsigned long fsbase;
>>>> +
>>>> +     if (task == current) {
>>>> +             fsbase = read_fsbase();
>>>> +     } else {
>>>> +             /*
>>>> +              * XXX: This will not behave as expected if called
>>>> +              * if fsindex != 0. This preserves an existing bug
>>>> +              * that will be fixed.
>>> 
>>> I'm late to this party, but let me ask the obvious question:
>>> 
>>>    Why is the existing bug not fixed as the first patch in the series?
>> 
>> IIRC that was how I did it in the old version of this code.  I think
>> it did it because it was less messy to fix the bug after cleaning up
>> the code, but I could be remembering wrong.
> 
> Fair enough. Though the general rule is: Fix bugs first and then do
> features, unless you really need the extra step to fix it proper.
> 
> Now in that case the real question is whether this is a bug or just a
> slight incorrectness which has no practical impact. If it's the latter,
> then introduce the new function which does the right thing first and make
> the new fs/gs base functions use it without having a blurb about preserving
> bugs.

The idea was to have one patch that was intended to have no observable effect (pure refactor) and another to change behavior in an easily reviewable way.  I should probably not have used the word bug :)

> 
> Thanks,
> 
>    tglx
> 
> 
>