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

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Message-ID: <alpine.LFD.2.02.1206041519430.3086@ionos>
Date:	Mon, 4 Jun 2012 15:22:51 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jan Kiszka <jan.kiszka@...mens.com>
cc:	Avi Kivity <avi@...hat.com>,
	Alex Williamson <alex.williamson@...hat.com>,
	"kvm@...r.kernel.org" <kvm@...r.kernel.org>,
	"mtosatti@...hat.com" <mtosatti@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"yongjie.ren@...el.com" <yongjie.ren@...el.com>
Subject: Re: [PATCH] KVM: Use IRQF_ONESHOT for assigned device MSI
 interrupts

On Mon, 4 Jun 2012, Jan Kiszka wrote:

> On 2012-06-04 15:07, Thomas Gleixner wrote:
> > On Mon, 4 Jun 2012, Jan Kiszka wrote:
> >> On 2012-06-04 13:21, Thomas Gleixner wrote:
> >> So this shortcut requires some checks before being applied to a specific
> >> MSI/MSI-X vector.
> >>
> >>
> >> Taking KVM aside, my general question remains if threaded MSI handlers
> >> of all devices really need to apply IRQF_ONESHOT though they should have
> >> no use for it.
> > 
> > In theory no, but we had more than one incident, where threaded irqs
> > w/o a primary handler and w/o IRQF_ONEHSOT lead to full system
> > starvation. Linus requested this sanity check and I think it's sane
> > and required.
> 
> OK.
> 
> > 
> > In fact it's a non issue for MSI. MSI uses handle_edge_irq which does
> > not mask the interrupt. IRQF_ONESHOT is a noop for that flow handler.
> 
> Isn't irq_finalize_oneshot processes for all flows?

Right, forgot about that. The only way we can avoid that, is that we
get a hint from the underlying irq chip/ handler setup with an extra
flag to tell the core, that it's safe to avoid the ONESHOT/finalize
magic.

Thanks,

	tglx
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