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

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Message-ID: <20100916094324.GE20864@redhat.com>
Date:	Thu, 16 Sep 2010 11:43:24 +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 11:25:53AM +0200, Gleb Natapov wrote:
> On Thu, Sep 16, 2010 at 11:10:55AM +0200, Michael S. Tsirkin wrote:
> > On Thu, Sep 16, 2010 at 11:02:56AM +0200, Gleb Natapov wrote:
> > > On Wed, Sep 15, 2010 at 08:54:17PM +0200, Michael S. Tsirkin wrote:
> > > > To avoid bouncing irqfd injection out to workqueue context,
> > > > we must support injecting irqs from local interrupt
> > > > context. Doing this seems to only require disabling
> > > > irqs locally.
> > > > 
> > > > RFC, completely untested, x86 only.
> > > > 
> > > > Thoughts?
> > > > 
> > > We do not want to disable irqs for a long time and some of code paths
> > > under lock involve looping over all cpus. For MSI injection path is
> > > lockless and this is the only case that matters,
> > 
> > MSI only appeared in rhel6, older guests still use level interrupts.
> So they are already slow for other reasons.

They are slower than MSI, but I do not want irqfd to be slower
than ioctls.

> > Which paths require looping over all cpus? Do PCI interrupts
> > need this?
> > 
> All interrupts need it. IOAPIC has a loop to find dst cpu for interrupt.
> Pic has a loop to find cpu in virtual wire mode.
> 
> --
> 			Gleb.
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