linux-kernel - Re: [PATCH] x86: Don't ack_APIC_irq() if lapic is disabled in GENERIC_INTERRUPT

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Date:	Sun, 20 Sep 2009 20:21:24 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Sheng Yang <sheng@...ux.intel.com>,
	Yinghai Lu <yinghai@...nel.org>,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	linux-kernel@...r.kernel.org, Dimitri Sivanich <sivanich@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [PATCH] x86: Don't ack_APIC_irq() if lapic is disabled in
	GENERIC_INTERRUPT_VECTOR handler


* Cyrill Gorcunov <gorcunov@...il.com> wrote:

> [Ingo Molnar - Sun, Sep 20, 2009 at 08:06:32PM +0200]
> | 
> | * Sheng Yang <sheng@...ux.intel.com> wrote:
> | 
> | > Otherwise would cause trouble...
> | > 
> | > Cc: Dimitri Sivanich <sivanich@....com>
> | > Signed-off-by: Sheng Yang <sheng@...ux.intel.com>
> | > ---
> | >  arch/x86/kernel/irq.c |    3 ++-
> | >  1 files changed, 2 insertions(+), 1 deletions(-)
> | > 
> | > diff --git a/arch/x86/kernel/irq.c b/arch/x86/kernel/irq.c
> | > index b0cdde6..78b23d0 100644
> | > --- a/arch/x86/kernel/irq.c
> | > +++ b/arch/x86/kernel/irq.c
> | > @@ -257,7 +257,8 @@ void smp_generic_interrupt(struct pt_regs *regs)
> | >  {
> | >  	struct pt_regs *old_regs = set_irq_regs(regs);
> | >  
> | > -	ack_APIC_irq();
> | > +	if (!disable_apic)
> | > +		ack_APIC_irq();
> | 
> | Wont in that case the apic->ack method be a NOP?
> | 
> | 	Ingo
> | 
> 
> iirc it was Xen related patch. So it's not that simple.
> 
> I've pointed out Sheng about disable_apic. I'm not Xen
> specialist but Xen team seem to use specific apic setup
> so our "dummy" operations are not involved (case they
> set disable_apic=1 without "turn off" apic ops in real).
> Something like that.

They should then set a NOP function in that case. We really dont want to 
slow down hotpath functions like smp_generic_interrupt() with flaggery.

	Ingo
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