Re: [PATCH RFC] kvm: enable irq injection from interrupt context

From: Gleb Natapov
Date: Thu Sep 16 2010 - 06:20:53 EST


On Thu, Sep 16, 2010 at 12:13:39PM +0200, Michael S. Tsirkin wrote:
> On Thu, Sep 16, 2010 at 12:13:32PM +0200, Gleb Natapov wrote:
> > On Thu, Sep 16, 2010 at 11:53:10AM +0200, Michael S. Tsirkin wrote:
> > > On Thu, Sep 16, 2010 at 11:46:03AM +0200, Avi Kivity wrote:
> > > > On 09/16/2010 11:25 AM, Gleb Natapov wrote:
> > > > >>
> > > > >> MSI only appeared in rhel6, older guests still use level interrupts.
> > > > >So they are already slow for other reasons.
> > > > >
> > > >
> > > > Exactly, for example they need to exit to userspace to ack the
> > > > interrupt. That's far slower than the workqueue.
> > >
> > > Well, this is not exactly comparable: you might get
> > > same irq asserted multiple times and only deasserted once.
> > >
> > Are we talking about level interrupts? Why would you assert level
> > triggered interrupt multiple times before deasserting it?
>
> User of irqfd has no way to know what current interrupt level is.
> So it has to keep asserting.
>
Why can't it keep track of current level?

--
Gleb.
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