linux-kernel - Re: [PATCH] x86,perf: Unmask LVTPC only if we have APIC supported

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Date:	Sat, 13 Mar 2010 15:32:56 +0300
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Lin Ming <ming.m.lin@...el.com>,
	Peter Zijlstra <peterz@...radead.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] x86,perf: Unmask LVTPC only if we have APIC supported

On Sat, Mar 13, 2010 at 01:24:32PM +0100, Ingo Molnar wrote:
> 
> * Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
> 
> > Ingo reported
> > |
> > | There's a build failure on -tip with the P4 driver, on UP 32-bit, if
> > | PERF_EVENTS is enabled but UP_APIC is disabled:
> > |
> > | arch/x86/built-in.o: In function `p4_pmu_handle_irq':
> > | perf_event.c:(.text+0xa756): undefined reference to `apic'
> > | perf_event.c:(.text+0xa76e): undefined reference to `apic'
> > |
> > 
> > So we have to unmask LVTPC only if we're configured to have one.
> > 
> > Reported-by: Ingo Molnar <mingo@...e.hu>
> > CC: Lin Ming <ming.m.lin@...el.com>
> > CC: Peter Zijlstra <peterz@...radead.org>
> > Signed-off-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> > ---
> >  arch/x86/kernel/cpu/perf_event_p4.c |    2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > Index: linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
> > =====================================================================
> > --- linux-2.6.git.orig/arch/x86/kernel/cpu/perf_event_p4.c
> > +++ linux-2.6.git/arch/x86/kernel/cpu/perf_event_p4.c
> > @@ -365,8 +365,10 @@ static int p4_pmu_handle_irq(struct pt_r
> >  	}
> >  
> >  	if (handled) {
> > +#ifdef CONFIG_X86_LOCAL_APIC
> >  		/* p4 quirk: unmask it again */
> >  		apic_write(APIC_LVTPC, apic_read(APIC_LVTPC) & ~APIC_LVT_MASKED);
> > +#endif
> >  		inc_irq_stat(apic_perf_irqs);
> 
> This ugly #ifdef looks like a workaround though. Why doesnt apic_write() map 
> to nothing in that case?
> 
> 	Ingo
> 

It is. I mean -- it maps to nothing if apic is disabled. But the scenario is
that no apic configured at all. Actually I wonder how this code is supposed to
work without apic support.

Pehpaps better to make a p4 quirk helper here, since #ifdef at this point looks
ugly indeed.

Don't apply it then. Will back with other solution.

	-- Cyrill
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