linux-kernel - Re: [PATCH v3 2/4] genirq: Inform handler about line sharing state

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Message-ID: <4D0B3C11.4090307@siemens.com>
Date:	Fri, 17 Dec 2010 11:31:45 +0100
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Thomas Gleixner <tglx@...utronix.de>
CC:	Jan Kiszka <jan.kiszka@....de>, Avi Kivity <avi@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm <kvm@...r.kernel.org>, Tom Lyon <pugs@...co.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"Michael S. Tsirkin" <mst@...hat.com>
Subject: Re: [PATCH v3 2/4] genirq: Inform handler about line sharing state

Am 17.12.2010 11:23, Thomas Gleixner wrote:
> On Fri, 17 Dec 2010, Jan Kiszka wrote:
>> Am 16.12.2010 21:26, Jan Kiszka wrote:
>>> Am 16.12.2010 14:13, Thomas Gleixner wrote:
>>>> On Mon, 13 Dec 2010, Jan Kiszka wrote:
>>>>> +	if (old_action && (old_action->flags & IRQF_ADAPTIVE) &&
>>>>> +	    !(desc->irq_data.drv_status & IRQS_SHARED)) {
>>>>> +		/*
>>>>> +		 * Signal the old handler that is has to switch to shareable
>>>>> +		 * handling mode. Disable the line to avoid any conflict with
>>>>> +		 * a real IRQ.
>>>>> +		 */
>>>>> +		disable_irq(irq);
>>>>
>>>> This is weird, really. I thought you wanted to avoid waiting for the
>>>> threaded handler to finish if it's on the fly. So this should be
>>>> disable_irq_nosync() or did you change your mind ?
>>>
>>> No, I did not. I wanted to avoid that we set MAKE_SHAREABLE while there
>>> might be another IRQ in flight. The handler that is called due to a real
>>> IRQ might misinterpret MAKE_SHAREABLE as "there is no real event" and
>>> perform the wrong steps (at least the current implementation for KVM would).
>>
>> Actually, the requirement we have to fulfill here is to avoid that the
>> hardirq handler sees !SHARED while the threaded one reads "SHARED". To
>> achieve this without disabling the line, I'm still searching for a way
>> to couple the sharing state of associated hard and threaded handler runs
>> - but I think there is no reliable association, is there?
> 
> Unfortunately not. So the only way to solve that is disabling the
> interrupt which makes sure that all handlers have completed.

Hmm, what a pity.

> 
> OTOH, if we have to disable anyway, then we could simply keep it
> disabled across the installation of a new handler. That would make the
> notification business go away, wouldn't it ?

No, the notification is still necessary in case the registered handler
keeps the line off after returning from both hard and threaded handler.

Jan

-- 
Siemens AG, Corporate Technology, CT T DE IT 1
Corporate Competence Center Embedded Linux
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