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Date: Sun, 20 Sep 2009 22:49:06 +0400
From: Cyrill Gorcunov <gorcunov@gmail.com>
To: Ingo Molnar <mingo@elte.hu>, Sheng Yang <sheng@linux.intel.com>,
       Yinghai Lu <yinghai@kernel.org>,
       Suresh Siddha <suresh.b.siddha@intel.com>, linux-kernel@vger.kernel.org,
       Dimitri Sivanich <sivanich@sgi.com>,
       Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>
Subject: Re: [PATCH] x86: Don't ack_APIC_irq() if lapic is disabled in
	GENERIC_INTERRUPT_VECTOR handler
Message-ID: <20090920184906.GE32176@lenovo>
References: <1252403546-12544-1-git-send-email-sheng@linux.intel.com> <20090920180632.GB8498@elte.hu> <20090920181326.GB32176@lenovo> <20090920182124.GD8498@elte.hu> <20090920183011.GC32176@lenovo> <20090920184203.GD32176@lenovo>
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[Cyrill Gorcunov - Sun, Sep 20, 2009 at 10:42:03PM +0400]
| [Cyrill Gorcunov - Sun, Sep 20, 2009 at 10:30:11PM +0400]
| ...
| | | > 
| | | > 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
| 
| Wait a bit Ingo, please. It seems I'm having different
| patch series in mind. Need to restore mail thread.
| Will back soon :)
| 
| 	-- Cyrill

yeah, it comes from Xen RFC series. Here is a quote from
conversation.

> Sheng Yan
>
> | | is there was some problem with it? I'm asking you
> | | because if disable_apic=1 then any apic write/read
> | | operations become NOPs. So I don't see how it may
> | | hurt. But I could be missing something.
> | |
> | |   -- Cyrill
> |
> | Ah, I see -- it's due to your other patch...
> | Hmm this makes all "disable apic" idea less
> | general. And safety of ack_APIC_irq is now
> | under suspicious.
>
> Um, probably. I've seen a ack_APIC_irq() in do_IRQ when handle_irq() fail.
> Seems the assumption that ack_APIC_irq() always safe is there. I will check if
> I can make it more elegant - maybe disable the warning in the Xen code...
>

Personally, I think "out-of-xen-thread" this patch is not needed.
And if this apic-ack operation causes any kind of problems --
this problem should be fixed without disable_apic involved.

	-- Cyrill
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