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

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Date:	Thu, 16 Sep 2010 12:44:55 +0200
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Gleb Natapov <gleb@...hat.com>
Cc:	Avi Kivity <avi@...hat.com>, Marcelo Tosatti <mtosatti@...hat.com>,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH RFC] kvm: enable irq injection from interrupt context

On Thu, Sep 16, 2010 at 12:20:47PM +0200, Gleb Natapov wrote:
> 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?

This breaks the model: eventfd user is unaware of PCI, levels and such:
it just signals the event.  Remember that asserts are done from e.g. vhost-net,
deasserts need to be handled by qemu.

> --
> 			Gleb.
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