linux-kernel - Re: [PATCH] x86: fix dummy apic read/write warning

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Message-ID: <20090607070136.GA4547@lenovo>
Date:	Sun, 7 Jun 2009 11:01:36 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Yinghai Lu <yinghai@...nel.org>
Cc:	Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86: fix dummy apic read/write warning

[Yinghai Lu - Sat, Jun 06, 2009 at 03:16:38PM -0700]
| ingo got
| [    0.000000] Using APIC driver default
| [    0.000000] SMP: Allowing 1 CPUs, 0 hotplug CPUs
| [    0.000000] Local APIC disabled by BIOS -- you can enable it with "lapic"
| [    0.000000] APIC: disable apic facility
| [    0.000000] ------------[ cut here ]------------
| [    0.000000] WARNING: at arch/x86/kernel/apic/apic.c:254 native_apic_read_dummy+0x2d/0x3b()
| [    0.000000] Hardware name: HP OmniBook PC
| 
| we have wrong check in dummy read and write.
| [we only test on system that have APIC with "disableapic"...]
| 
| Signed-off-by: yinghai Lu <yinghai@...nel.org>
| 
| ---
|  arch/x86/kernel/apic/apic.c |    4 ++--
|  1 file changed, 2 insertions(+), 2 deletions(-)
| 
| Index: linux-2.6/arch/x86/kernel/apic/apic.c
| ===================================================================
| --- linux-2.6.orig/arch/x86/kernel/apic/apic.c
| +++ linux-2.6/arch/x86/kernel/apic/apic.c
| @@ -246,12 +246,12 @@ static int modern_apic(void)
|   */
|  static void native_apic_write_dummy(u32 reg, u32 v)
|  {
| -	WARN_ON_ONCE((cpu_has_apic || !disable_apic));
| +	WARN_ON_ONCE((cpu_has_apic && !disable_apic));
|  }
|  
|  static u32 native_apic_read_dummy(u32 reg)
|  {
| -	WARN_ON_ONCE((cpu_has_apic || !disable_apic));
| +	WARN_ON_ONCE((cpu_has_apic && !disable_apic));
|  	return 0;
|  }
|  
| 

No Yinghai, this is wrong. native_apic_write_dummy is not
admissible to be called after apic->write has been NOP'ified.

If a site calls for apic->write after apic_disable() it's
a plain *bug* in caller. So we should continute warning if
cpu_has_apic _or_ !disable_apic.

Though since for SMP compiled kernel we still rely on
apic->read the snippet is to be changed to (cpu_has_apic && !disable_apic)
indeed.

	-- Cyrill
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