linux-kernel - Re: [patch 1/3] x86/cpu: Remove segment load from switch_to_new

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Date:   Mon, 18 Jul 2022 20:55:03 +0200
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        the arch/x86 maintainers <x86@...nel.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Andrew Cooper <Andrew.Cooper3@...rix.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Johannes Wikner <kwikner@...z.ch>,
        Alyssa Milburn <alyssa.milburn@...ux.intel.com>,
        Jann Horn <jannh@...gle.com>, "H.J. Lu" <hjl.tools@...il.com>,
        Joao Moreira <joao.moreira@...el.com>,
        Joseph Nuzman <joseph.nuzman@...el.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Juergen Gross <jgross@...e.com>,
        "Peter Zijlstra (Intel)" <peterz@...radead.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>
Subject: Re: [patch 1/3] x86/cpu: Remove segment load from switch_to_new_gdt()

On Mon, Jul 18 2022 at 11:43, Linus Torvalds wrote:
> So I appreciate the added big comments in this code, but looking at this patch:
> On Mon, Jul 18, 2022 at 10:52 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> +        * For secondary CPUs this is not a problem because they start
>> +        * already with the direct GDT and the real GSBASE. This invocation
>> +        * is pointless and will be removed in a subsequent step.
>> +        */
>> +       if (IS_ENABLED(CONFIG_X86_64))
>> +               wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
>>  }
>
> ... while those comments are nice and all, I do think this retains the
> basic insanity of having "switch_to_new_gdt()" do magical things on
> x86-64 that don't really match the name.
>
> So honestly, I'd be happier of that whole
>
>        if (IS_ENABLED(CONFIG_X86_64))
>                wrmsrl(MSR_GS_BASE, cpu_kernelmode_gs_base(cpu));
>
> was migrated to the callers instead. There aren't *that* many callers.
>
> I expect that it is then quite possible that several of the call-sites
> would go "GS_BASE is already correct here, I can remove this".

With the next patch we have only two left. The SMP and the UP case. Let
me look whether the UP needs it at all.

> But even if every single caller keeps that wrmsrl() around, at least
> it wouldn't be hidden behind a function call that has a name that
> implies something completely different is happening.
>
> And no, I don't care *that* deeply, so this is just a suggestion.
>
> But wouldn't it be nice if this function was actually named by what it
> does, rather than by what it used to do back in the i386 days when the
> GDT affected the segment bases?

Yes. Let me come up with a sensible name.

Thanks,

        tglx