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

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Date:	Fri, 01 Jun 2012 19:14:15 +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 19:03, Alex Williamson wrote:
> On Fri, 2012-06-01 at 18:39 +0200, Jan Kiszka wrote:
>> On 2012-06-01 18:16, Alex Williamson wrote:
>>> The kernel no longer allows us to pass NULL for a hard interrupt
>>> handler without IRQF_ONESHOT.  Should have been using this flag
>>> anyway.
>>
>> This make the IRQ handling tail a bit slower (due to
>> irq_finalize_oneshot). MSIs are edge-triggered, so there was no need for
>> masking in theory.
> 
> Aren't these asynchronous since we can theoretically do
> irq_finalize_oneshot while the guest is servicing the device?

If it runs on a different CPU. But usually it's more efficient to have
handler and user on the same CPU. And this work has to be processed
somewhere.

> 
>>  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?

Jan

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