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 22:49:06 +0400
From:	Cyrill Gorcunov <gorcunov@...il.com>
To:	Ingo Molnar <mingo@...e.hu>, 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 - 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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