linux-kernel - Re: [PATCH 0/5] Threaded MSI interrupt for VFIO PCI device

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Message-ID: <20160106074207.GB15116@jnakajim-build>
Date:	Tue, 5 Jan 2016 23:42:07 -0800
From:	Yunhong Jiang <yunhong.jiang@...ux.intel.com>
To:	Paolo Bonzini <pbonzini@...hat.com>
Cc:	Alex Williamson <alex.williamson@...hat.com>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] Threaded MSI interrupt for VFIO PCI device

On Wed, Dec 16, 2015 at 10:55:12PM +0100, Paolo Bonzini wrote:
> 
> 
> On 16/12/2015 20:15, Alex Williamson wrote:
> > The consumers would be, for instance, Intel PI + the threaded handler
> > added in this series.  These run independently, the PI bypass simply
> > makes the interrupt disappear from the host when it catches it, but if
> > the vCPU isn't running in the right place at the time of the interrupt,
> > it gets delivered to the host, in which case the secondary consumer
> > implementing handle_irq() provides a lower latency injection than the
> > eventfd path.  If PI isn't supported, only this latter consumer is
> > registered.
> 
> I would implement the two in a single consumer, knowing that only one of
> the two parts would effectively run.  But because of the possibility of
> multiple consumers implementing handle_irq(), I am not sure if this is
> feasible.

So is it possible that we limit only one consumer with handle_irq(), as my 
previous response to Alex? We can extend it in future if we do need support 
multiple consumder implementing handle_irq()?

Thanks
--jyh

> 
> > On the surface it seems like a reasonable solution, though having
> > multiple consumers implementing handle_irq() seems problematic.  Do we
> > get multiple injections if we call them all?
> 
> Indeed.
> 
> > Should we have some way
> > to prioritize one handler versus another?  Perhaps KVM should have a
> > single unified consumer that can provide that sort of logic, though we
> > still need the srcu code added here to protect against registration and
> > irq_handler() races.  Thanks,
> 
> I'm happy to see that we have the same doubts. :)
> 
> Paolo
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