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Subject: Re: [PATCH v4 1/7] x86/fsgsbase/64: Introduce FS/GS base helper functions
From:   Andy Lutomirski <luto@amacapital.net>
In-Reply-To: <alpine.DEB.2.21.1806221735400.2402@nanos.tec.linutronix.de>
Date:   Fri, 22 Jun 2018 09:08:59 -0700
Cc:     Andy Lutomirski <luto@kernel.org>,
        "Bae, Chang Seok" <chang.seok.bae@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@kernel.org>,
        Andi Kleen <ak@linux.intel.com>,
        Dave Hansen <dave.hansen@linux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@intel.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        LKML <linux-kernel@vger.kernel.org>
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Message-Id: <851713FF-90E1-434A-BC31-5DE437AB530E@amacapital.net>
References: <1529536506-26237-1-git-send-email-chang.seok.bae@intel.com> <1529536506-26237-2-git-send-email-chang.seok.bae@intel.com> <alpine.DEB.2.21.1806221610500.2402@nanos.tec.linutronix.de> <CALCETrVzLT0AwUFxZGjuREaontqzJe0DEccNFwLTKdLcLxcARA@mail.gmail.com> <alpine.DEB.2.21.1806221735400.2402@nanos.tec.linutronix.de>
To:     Thomas Gleixner <tglx@linutronix.de>
Sender: linux-kernel-owner@vger.kernel.org
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> On Jun 22, 2018, at 8:39 AM, Thomas Gleixner <tglx@linutronix.de> wrote:
>=20
>> On Fri, 22 Jun 2018, Andy Lutomirski wrote:
>> On Fri, Jun 22, 2018 at 7:28 AM Thomas Gleixner <tglx@linutronix.de> wrot=
e:
>>>> +unsigned long read_task_fsbase(struct task_struct *task)
>>>> +{
>>>> +     unsigned long fsbase;
>>>> +
>>>> +     if (task =3D=3D current) {
>>>> +             fsbase =3D read_fsbase();
>>>> +     } else {
>>>> +             /*
>>>> +              * XXX: This will not behave as expected if called
>>>> +              * if fsindex !=3D 0. This preserves an existing bug
>>>> +              * that will be fixed.
>>>=20
>>> I'm late to this party, but let me ask the obvious question:
>>>=20
>>>    Why is the existing bug not fixed as the first patch in the series?
>>=20
>> 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.
>=20
> 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.
>=20
> 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 preservin=
g
> bugs.

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

>=20
> Thanks,
>=20
>    tglx
>=20
>=20
>=20