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

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Date:	Fri, 8 Jun 2012 17:39:27 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Jan Kiszka <jan.kiszka@...mens.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>, 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, Jun 04, 2012 at 01:40:28PM +0200, Jan Kiszka wrote:
> On 2012-06-04 13:21, Thomas Gleixner wrote:
> > On Sun, 3 Jun 2012, Avi Kivity wrote:
> > 
> >> On 06/01/2012 09:26 PM, Jan Kiszka wrote:
> >>>
> >>>> 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.
> >>>
> >>
> >> btw, I'm hoping we can unthread assigned MSIs.  If the delivery is
> >> unicast, we can precalculate everything and all the handler has to do is
> >> set the IRR, KVM_REQ_EVENT, and kick the vcpu.  All of these can be done
> >> from interrupt context with just RCU locking.
> > 
> > There is really no need to run MSI/MSI-X interrupts threaded for
> > KVM. I'm running the patch below for quite some time and it works like
> > a charm.
> > 
> > Thanks,
> > 
> > 	tglx
> > ----


....

> 
> This may work in practice but has two conceptual problems:
>  - we do not want to run a potential broadcast to all VCPUs to run in
>    a host IRQ handler

I'm not sure why this one is a problem: injecting an interrupt
once you know the vcpu seems really cheap.
It's true that scanning vcpus might take a bit more time
when there are lots of them but it's a single
linear scan that we do anyway.

And we also inject msi from irqfd callback with interrupts
disabled which seems equivalent.

Pls correct me if I'm wrong.

>  - crazy user space could have configured the route to end up in the
>    PIC or IOAPIC, and both are not hard-IRQ safe (this should probably
>    be caught on setup)

Yes this needs to be fixed up.

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