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

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Date:	Sat, 13 Mar 2010 13:24:32 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...nvz.org>
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


* 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
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