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

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Message-ID: <5671DDC0.6040507@redhat.com>
Date:	Wed, 16 Dec 2015 22:55:12 +0100
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Alex Williamson <alex.williamson@...hat.com>,
	Yunhong Jiang <yunhong.jiang@...ux.intel.com>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 0/5] Threaded MSI interrupt for VFIO PCI device



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.

> 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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