linux-kernel - Re: [PATCH] x86/apic: check FADT settings after enable x2apic

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Date:	Tue, 29 Jan 2013 08:47:46 +0100
From:	Ingo Molnar <mingo@...nel.org>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	"Wang, Song-Bo (Stoney)" <song-bo.wang@...com>,
	"H. Peter Anvin" <hpa@...or.com>, Ingo Molnar <mingo@...e.hu>,
	Thomas Gleixner <tglx@...utronix.de>,
	"Zhang, Lin-Bao (Linux Kernel R&D)" <linbao.zhang@...com>,
	"Pearson, Greg" <greg.pearson@...com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"suresh.b.siddha@...el.com" <suresh.b.siddha@...el.com>
Subject: Re: [PATCH] x86/apic: check FADT settings after enable x2apic


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

> On Mon, Jan 28, 2013 at 2:11 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >
> >> HP has systems that work with x2apic phys mode and BIOS set
> >> ACPI_FADT_APIC_PHYSICAL in FADT table, and all cpuid < 255,
> >> the spec requires BIOS only put system on xapic mode. Kernel
> >
> > Which spec?
> >
> >> will set to x2apic logical mode instead of x2apic phys mode.
> >
> > Which has exactly what bad effect on users of these systems?
> >
> > You left out the most important information from the changelog:
> > why do users care, what good does the patch do?
> 
> please check you are happy with this:
> 
> ---
> From:   Stoney Wang <song-bo.wang@...com>
> Subject: [PATCH] x86, apic: Check fadt x2apic phys in x2apic_phys_probe()
> 
> HP has systems that only work with x2apic phys mode and BIOS set
> ACPI_FADT_APIC_PHYSICAL in FADT table. But all apicid < 255,
> according to x2apic-spec, chapter 2.9, BIOS need to pass the control
> to the OS with xapic mode.
> Kernel will set apic driver wrong to x2apic cluster instead of x2apic phys.
> 
> The user will have to append nox2apic in boot command line to stay xapic mode,
> or append x2apic_phys to switch to x2apic phys mode.

This still does not explain what happens if none of this user 
action is taken. I.e. what exact _user visible problem_ does the 
patch fix?

Is this really so unimportant to you? Almost everyone will start 
a changelog with explaining what badness happens. Not you - you 
explain everything from how the fix works to how to work around 
the bug - except describing the most important thing: theuser 
visible problem itself ... Weird.

Thanks,

	Ingo
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