linux-kernel - Re: [KVM PATCH v7 2/3] kvm: add support for irqfd via eventfd-notification interface

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Message-ID: <alpine.DEB.1.10.0905142016590.7574@makko.or.mcafeemobile.com>
Date:	Thu, 14 May 2009 20:22:20 -0700 (PDT)
From:	Davide Libenzi <davidel@...ilserver.org>
To:	Gregory Haskins <ghaskins@...ell.com>
cc:	Avi Kivity <avi@...hat.com>, kvm@...r.kernel.org,
	viro@...IV.linux.org.uk,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [KVM PATCH v7 2/3] kvm: add support for irqfd via eventfd-notification
 interface

On Thu, 14 May 2009, Gregory Haskins wrote:

> Avi Kivity wrote:
> > Gregory Haskins wrote:
> >> KVM provides a complete virtual system environment for guests, including
> >> support for injecting interrupts modeled after the real
> >> exception/interrupt
> >> facilities present on the native platform (such as the IDT on x86).
> >> Virtual interrupts can come from a variety of sources (emulated devices,
> >> pass-through devices, etc) but all must be injected to the guest via
> >> the KVM infrastructure.  This patch adds a new mechanism to inject a
> >> specific
> >> interrupt to a guest using a decoupled eventfd mechnanism:  Any legal
> >> signal
> >> on the irqfd (using eventfd semantics from either userspace or
> >> kernel) will
> >> translate into an injected interrupt in the guest at the next available
> >> interrupt window.
> >>
> >> +
> >> +static void
> >> +irqfd_inject(struct work_struct *work)
> >> +{
> >> +    struct _irqfd *irqfd = container_of(work, struct _irqfd, work);
> >> +    struct kvm *kvm = irqfd->kvm;
> >> +
> >>   
> >
> >
> > I think you need to ->read() from the irqfd, otherwise the count will
> > never clear.
> 
> Yeah, and this is a disavantage to using eventfd vs a custom anon-fd
> implementation.
> 
> However, the count is really only there for deciding whether to sleep a
> traditional eventfd recipient which doesn't really apply in this
> application.  I suppose we could try to invoke the read method (or add a
> new method to eventfd to allow it to be cleared independent of the
> f_ops->read() (ala eventfd_signal() vs f_ops->write()).  I'm not
> convinced we really need to worry about it, though.  IMO we can just let
> the count accumulate.
> 
> But if you insist this loose end should be addressed, perhaps Davide has
> some thoughts on how to best do this?

The counter is 64bit, so at 1M IRQ/s will take about 585K years to 
saturate. But from a symmetry POV, it may be better to clear it. Maybe 
with a kernel-side eventfd_read()?


- Davide


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