From mboxrd@z Thu Jan  1 00:00:00 1970
From: Jeremy Fitzhardinge <jeremy@goop.org>
Subject: Re: radeon in dom0/ivtv in domU: irq 16 nobody cared
Date: Tue, 13 Apr 2010 12:21:41 -0700
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To: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: xen-devel@lists.xensource.com, Mark Hurenkamp <mark.hurenkamp@xs4all.nl>
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On 04/13/2010 12:10 PM, Konrad Rzeszutek Wilk wrote:
>> Does the kernel get to know about the passed-through irqs?  I was
>> thinking that at pass-through time it would install the handler in dom0
>> on the irq (and all other domains sharing the irq), and then the handler
>> could do that hypercall and return IRQ_HANDLED / IRQ_NONE accordingly.
>>     
> That was my other thought. But that means that the handler has to
> do a hypercall - which I thought is a bit too heavy handed considering we only need
> to know whether it is shared only once.

It could just return IRQ_HANDLED unconditionally, without doing the
hypercall.  Remember you also need it in the domU and any other domU
using the same GSI.

>  The fake irq handler gets
> disabled if the domain is turned off/pci hotplug removed.
>   

Yep.

> Currently the fake IRQ handler is installed on the IRQ that is requested
> by the guest. So if the IRQ is not shared in Dom0, it still gets
> inserted in (and it won't receive any interrupts since that IRQ line is
> not shared with the domain 0).
>   

Just for clarity's sake, you mean GSI for each instance of IRQ here, right?

>> Hrm.  You could instrument Xen to see who's masking the interrupt.
>>     
> <nods>Right now I am, digging through code to figure out how to make this
> happen without inserting to much code in the handler which would have
> slowed the machine to a crawl.
>   

I was thinking of something very dumb and ad-hoc, like putting an
explicit "if (gsi == X && masking) print something".

    J