linux-kernel - Re: [PATCH v6 8/8] x86/vdso: Move out the CPU initialization

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Message-ID: <20181008083624.GB117675@gmail.com>
Date:   Mon, 8 Oct 2018 10:36:24 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Chang S. Bae" <chang.seok.bae@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        "H . Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Markus T Metzger <markus.t.metzger@...el.com>,
        Ravi Shankar <ravi.v.shankar@...el.com>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 8/8] x86/vdso: Move out the CPU initialization

* Chang S. Bae <chang.seok.bae@...el.com> wrote:

> The CPU and node number will be written, as early enough,
> to the segment limit of per CPU data and TSC_AUX MSR entry.
> The information has been retrieved by vgetcpu in user space
> and will be also loaded from the paranoid entry, when
> FSGSBASE enabled.
>
> The new setup function is named after the getcpu(2) system
> call, and will be called during each CPU initialization
> (before setting up IST). It makes a facility useful to both
> the kernel and userspace unconditionally available much
> sooner.
> 
> The change brings a substantial code removal. The redundant
> setting of the segment in entry/vdso/vma.c and hotplug
> notifier are removed.

The title and the changelog is totally unreadable, full of grammar errors
which makes it actively misleading...

A good changelog should explain not what it does, but _why_ it is
done:

  x86/vdso: Initialize the CPU/node NR segment descriptor earlier

  Currently the CPU/node NR segment descriptor (GDT_ENTRY_CPU_NUMBER) is
  initialized relatively late during CPU init, from the vCPU code, which
  has a number of disadvantages, such as hotplug CPU notifiers and SMP
  cross-calls.

  Instead just initialize it much earlier, directly in cpu_init().

  This reduces complexity and increases robustness.

I've edited the changelog, but please keep this in mind for future submissions.

I also made a number of other cleanups to the code, will push them out
after some testing.

Thanks,

	Ingo