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

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Date:	Sun, 7 Jun 2009 12:28:07 -0700
From:	Yinghai Lu <yhlu.kernel@...il.com>
To:	Cyrill Gorcunov <gorcunov@...il.com>
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

On Sun, Jun 7, 2009 at 12:01 AM, Cyrill Gorcunov<gorcunov@...il.com> wrote:
> [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.

ok, just change it to

     WARN_ON_ONCE(1);

there.

>
> 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.

YH
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