linux-kernel - Re: [PATCH 8/8] x86_64: Support for new UV apic

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Date:	Fri, 28 Mar 2008 15:24:30 -0500
From:	Jack Steiner <steiner@....com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	tglx@...utronix.de, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 8/8] x86_64: Support for new UV apic

On Fri, Mar 28, 2008 at 09:15:32PM +0100, Ingo Molnar wrote:
> 
> * Jack Steiner <steiner@....com> wrote:
> 
> > Index: linux/arch/x86/kernel/apic_64.c
> > ===================================================================
> > --- linux.orig/arch/x86/kernel/apic_64.c	2008-03-28 13:00:22.000000000 -0500
> > +++ linux/arch/x86/kernel/apic_64.c	2008-03-28 13:06:12.000000000 -0500
> > @@ -738,6 +738,7 @@ void __cpuinit setup_local_APIC(void)
> >  	unsigned int value;
> >  	int i, j;
> >  
> > +	preempt_disable();
> >  	value = apic_read(APIC_LVR);
> >  
> >  	BUILD_BUG_ON((SPURIOUS_APIC_VECTOR & 0x0f) != 0x0f);
> > @@ -831,6 +832,7 @@ void __cpuinit setup_local_APIC(void)
> >  	else
> >  		value = APIC_DM_NMI | APIC_LVT_MASKED;
> >  	apic_write(APIC_LVT1, value);
> > +	preempt_enable();
> >  }
> 
> hm, this looks a bit weird - why are all the preempt-disable/enable 
> calls needed?

The first patch had a preempt disable/enable in the function
that reads apicid (see read_apic_id() in arch/x86/kernel/genapic_64.c).
This seemed necessary since large system generate an apicid by reading
the live id & concatenating it with extra bits.

One of the review comments suggested that I change the preempt to a WARN()
since reading apic_id really should be done with preemtion disabled. The
added code eliminates the warnings.


--- jack
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