linux-kernel - Re: [PATCH] x86, apic: Check fadt x2apic phys in x2apic_phys

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Message-ID: <20130206131655.GH8696@gmail.com>
Date:	Wed, 6 Feb 2013 14:16:55 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org,
	Stoney Wang <song-bo.wang@...com>, stable@...nel.org
Subject: Re: [PATCH] x86, apic: Check fadt x2apic phys in x2apic_phys_probe()

* Yinghai Lu <yinghai@...nel.org> wrote:

> On Tue, Feb 5, 2013 at 4:24 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> > * Yinghai Lu <yinghai@...nel.org> wrote:
> >
> >> From: Stoney Wang <song-bo.wang@...com>
> >>
> >> When HP ProLiant DL980 G7 Server boot without x2apic_phys, there will be
> >> intermittent lost interrupts and could result in a hang or data loss.
> >
> > What does 'boot without x2apic_phys' mean?
> >
> > Does it mean x2apic_phys=0 boot command line? Or, because
> > x2apic_phys is off by default, does it simply mean that if it's
> > booted with a default kernel, without any workaround specified
> > on the boot command line?
> 
> means that user does not append "x2apic_phys" in boot command line.

The user does not append a whole lot of other 
behavior-modification command line options either! There's no 
apic=0 line either. Nor smp=0.

Adding this essentially irrelevant piece of information to the 
*FIRST*, most important sentence of the changelog is thus not 
just confusing but utterly misleading. Communications 101.

Instead it should say something like:

   When a HP ProLiant DL980 G7 Server boots a regular kernel,
   there will be intermittent lost interrupts which could
   result in a hang or (in extreme cases) data loss.

   The reason is that this system only supports x2apic physical 
   mode, while the kernel boots with a logical-cluster default 
   setting.

   This bug can be worked around by specifying the x2apic_phys=1
   boot option, but we want to handle this sytem without
   requiring manual workarounds.

Right? Writing a clean changelog is like writing clean code - 
you have to learn it if you want to contribute to the kernel 
smoothly. Think of it as an engineering task: the other required 
half of modifying kernel code.

Thanks,

	Ingo
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