linux-kernel - Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts

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Message-ID: <4FC90961.8030701@siemens.com>
Date:	Fri, 01 Jun 2012 20:26:41 +0200
From:	Jan Kiszka <jan.kiszka@...mens.com>
To:	Alex Williamson <alex.williamson@...hat.com>
CC:	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"avi@...hat.com" <avi@...hat.com>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yongjie.ren@...el.com" <yongjie.ren@...el.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>
Subject: Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI interrupts

On 2012-06-01 19:59, Alex Williamson wrote:
>>>>  Hmm, can't we trust the information that an IRQ
>>>> grabbed here is really a MSI type?
>>>
>>>
>>> Apparently not, comment added with this check (1c6c6952):
>>>
>>>        * The interrupt was requested with handler = NULL, so
>>>        * we use the default primary handler for it. But it
>>>        * does not have the oneshot flag set. In combination
>>>        * with level interrupts this is deadly, because the
>>>        * default primary handler just wakes the thread, then
>>>        * the irq lines is reenabled, but the device still
>>>        * has the level irq asserted. Rinse and repeat....
>>>        *
>>>        * While this works for edge type interrupts, we play
>>>        * it safe and reject unconditionally because we can't
>>>        * say for sure which type this interrupt really
>>>        * has. The type flags are unreliable as the
>>>        * underlying chip implementation can override them.
>>
>> I was talking about KVM here: Can't the KVM device assignment code
>> ensure that only MSIs are registered as such so that the above concerns
>> no longer apply?
> 
> Is that making assumptions about how the chipset handles an MSI?  Are

I suppose the nature of MSIs removes any need for assumptions about the
handling.

> you suggesting we need a request_edge_threaded_only_irq() API?  Thanks,

I'm just wondering if that restriction for threaded IRQs is really
necessary for all use cases we have. Threaded MSIs do not appear to me
like have to be handled that conservatively, but maybe I'm missing some
detail.

Jan

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