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 12:22:22 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        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>,
        "Ravi V. Shankar" <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 Wed, Oct 24, 2018 at 12:13 PM Bae, Chang Seok
<chang.seok.bae@...el.com> wrote:
>
> 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()?

Why should ..write_task() magically change the index but only if it's
writing current?

I think you should move it all the way out to the caller
(do_arch_prctl_64()?) and we can see if it makes sense there.

>
> Chang