linux-kernel - RE: [regression in -rc1] Re: [PATCH v6 2/8] x86/fsgsbase/64: Introduce FS/GS base helper functions

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Date:   Wed, 24 Oct 2018 19:13:32 +0000
From:   "Bae, Chang Seok" <chang.seok.bae@...el.com>
To:     Andy Lutomirski <luto@...nel.org>
CC:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        "Metzger, Markus T" <markus.t.metzger@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: RE: [regression in -rc1] Re: [PATCH v6 2/8] x86/fsgsbase/64:
 Introduce FS/GS base helper functions

On Tue, Sep 18, 2018 at 12:02 PM Andy Lutomirski <luto@...nel.org>
> On Tue, Sep 18, 2018 at 4:09 PM Chang S. Bae <chang.seok.bae@...el.com>
> wrote:
> >
> > With new helpers, FS/GS base access is centralized.
> > Eventually, when FSGSBASE instruction enabled, it will be faster.
> 
> Sorry for not catching this during review, but:
> 
> > +void x86_fsbase_write_cpu(unsigned long fsbase) {
> > +       /*
> > +        * Set the selector to 0 as a notion, that the segment base is
> > +        * overwritten, which will be checked for skipping the segment load
> > +        * during context switch.
> > +        */
> > +       loadseg(FS, 0);
> 
> ^^^
> 
> what?
> 
> > +       wrmsrl(MSR_FS_BASE, fsbase);
> > +}
> 
> I don't understand what the comment is trying to say, but the sole caller so far
> of this function is x86_gsbase_write_task(), and the code looks incorrect.
> 
> Ingo, I think we need to address this during this merge window, probably by
> removing the comment and the loadseg() call (and the same for
> gsbase...inactive).  But first, Chang, can you explain what exactly your intent is
> here?

It's coming from do_arch_prctl_64(). If you think it really makes confusion in 
x86_fsbase_write_cpu(), how about moving it to x86_fsbase_write_task()?

Chang