linux-kernel - Re: [PATCH -tip] x86: do_IRQ - send EOI for x86-32 on irq without handler v2

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Message-ID: <20090408154558.GC7556@lenovo>
Date:	Wed, 8 Apr 2009 19:45:58 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>
Subject: Re: [PATCH -tip] x86: do_IRQ - send EOI for x86-32 on irq without
	handler v2

[Ingo Molnar - Wed, Apr 08, 2009 at 04:52:18PM +0200]
| 
| * Cyrill Gorcunov <gorcunov@...nvz.org> wrote:
| 
| > Ingo, I think introducing additional dummy here would be a bit 
| > expencive -- a number of callers of ack_APIC_irq just should not 
| > check for disable_apic since it's not needed. disable_apic is to 
| > depend on CONFIG_X86_LOCAL_APIC rather then CONFIG_X86_64 
| > actually. So make it then. Or you mean something else? Like new 
| > apic->write_eoi operation? (I'm a bit tired so brain is a half 
| > functional now :)
| 
| no, i meant a dummy apic->write() method in the !apic case. Check 
| what ack_APIC_irq() does internally.
| 
| 	Ingo
|

Of course I saw how ack_APIC_irq implemented :) It's already guarded
by CONFIG_X86_LOCAL_APIC and I could just move check for disable_apic
right here (ie it could be like

static inline void ack_APIC_irq(void)
{
#ifdef CONFIG_X86_LOCAL_APIC
	/*
	 * ack_APIC_irq() actually gets compiled as a single instruction
	 * ... yummie.
	 */

	if (!disable_apic)
		/* Docs say use 0 for future compatibility */
		apic_write(APIC_EOI, 0);
#endif
}

but what is bothering me is that a number of ack_APIC_irq callers
will pass execution with always disable_apic=0 and as result
this checking would be just spedning cycles for free.

So Ingo, it seems I miss something on what you mean. apic->write is already
called only for CONFIG_X86_LOCAL_APIC here as well and the arguable
point is where to check for disable_apic variable. But do_IRQ is a
special case (wrt to say smp_apic_timer_interrupt or setup_local_APIC.
I mean as example -- setup_local_APIC is not even called for disable_apic=1).

/me: scratching the head heavily

        Cyrill
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