Re: [patch 1/3] x86/cpu: Remove segment load from switch_to_new_gdt()

Thomas Gleixner
2022-07-18T18:55:08Z

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@linutronix.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