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 22:30:11 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>
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

[Ingo Molnar - Sun, Sep 20, 2009 at 08:21:24PM +0200]
| 
| * 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
| 

Well, I suppose we should wait for Sheng's comments.
I wish I would answer you but I simply don't know Xen
code :)

	-- Cyrill
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