linux-kernel - Re: [PATCH] Make Intel 8-way Xeons boot again

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Date:	Sat, 9 Jan 2010 10:11:47 -0800 (PST)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ananth N Mavinakayanahalli <ananth@...ibm.com>
cc:	suresh.b.siddha@...el.com, yinghai@...nel.org,
	Ingo Molnar <mingo@...e.hu>,
	lkml <linux-kernel@...r.kernel.org>, stable@...nel.org
Subject: Re: [PATCH] Make Intel 8-way Xeons boot again

On Sat, 9 Jan 2010, Ananth N Mavinakayanahalli wrote:
>
> On an 8-way system with Intel Xeon X7350 CPUs, booting 2.6.32 or newer
> kernels fails at:
> 
> ...
> CPU0: Intel(R) Xeon(R) CPU           X7350  @ 2.93GHz stepping 0b
> Booting Node   0, Processors  #1 #2 #3 #4 #5 #6 #7 Ok.
> Brought up 8 CPUs
> Total of 8 processors activated (46906.05 BogoMIPS).
> 
> Git bisect showed 2fbd07a5f as the offending commit.

Ok, that commit definitely is buggy.

> With the patch below, I am able to boot the latest Linus' git tree on
> the machine. If this patch is correct, it needs to get into the stable
> tree too.

I don't think the patch is correct, though. The thing is, the AMD check 
seems to be the correct one: you can only use 'apic_flat' if all the APIC 
ID's are < 8.

It doesn't matter _how_ many CPU's you have. If you have two CPU's, but 
one of them has an APIC ID >= 8, then you cannot use the flat APIC model, 
since it depends on a 8-bit bitfield.

So your patch doesn't seem right either, because it still tests 
num_processors, which is bogus.

In fact, I can't for the life of me understand why it treats different 
vendors differently. Why is that code not just a simple

	/* Flat apic mode requires that all APIC ID's are in the range 0..7 */
	if (apic == &apic_flat && max_physical_apicid >= 8)
		apic = &apic_physflat;

instead, with no crazy vendor tests.

What am I missing? 

		Linus
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