linux-kernel - Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ

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Message-ID: <5631EAC8.1080805@redhat.com>
Date:	Thu, 29 Oct 2015 10:45:44 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>
Cc:	Yunhong Jiang <yunhong.jiang@...ux.intel.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, Marcelo Tosatti <mtosatti@...hat.com>
Subject: Re: [RFC PATCH] VFIO: Add a parameter to force nonthread IRQ



On 29/10/2015 04:11, Alex Williamson wrote:
> > The irqfd is already able to schedule a work item, because it runs with
> > interrupts disabled, so I think we can always return IRQ_HANDLED.
>
> I'm confused by this.  The problem with adding IRQF_NO_THREAD to our
> current handler is that it hits the spinlock that can sleep in
> eventfd_signal() and the waitqueue further down the stack before we get
> to the irqfd.  So if we split to a non-threaded handler vs a threaded
> handler, where the non-threaded handler either returns IRQ_HANDLED or
> IRQ_WAKE_THREAD to queue the threaded handler, there's only so much that
> the non-threaded handler can do before we start running into the same
> problem.

You're right.  I thought schedule_work used raw spinlocks (and then
everything would be done in the inject callback), but I was wrong.

Basically where irqfd_wakeup now does schedule_work, it would need to
return IRQ_WAKE_THREAD.  The threaded handler then can just do the
eventfd_signal.

Paolo

> I think that means that the non-threaded handler needs to
> return IRQ_WAKE_THREAD if we need to use the current eventfd_signal()
> path, such as if the bypass path is not available.  If we can get
> through the bypass path and the KVM irqfd side is safe for the
> non-threaded handler, inject succeeds and we return IRQ_HANDLED, right?
> Thanks,
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