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

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Message-ID: <CALCETrUr3DGwW8y6y_cffEKWO_KQs1J2_UvTBZsbdAzES_f-Ag@mail.gmail.com>
Date:   Wed, 24 Oct 2018 12:01:43 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     "Bae, Chang Seok" <chang.seok.bae@...el.com>
Cc:     Ingo Molnar <mingo@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andrew Lutomirski <luto@...nel.org>,
        "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: [regression in -rc1] Re: [PATCH v6 2/8] x86/fsgsbase/64: Introduce
 FS/GS base helper functions

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?