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Re: [Xen-devel] FW: about VIRQ & PIRQ

To: "Mark Williamson" <mark.williamson@xxxxxxxxxxxx>
Subject: Re: [Xen-devel] FW: about VIRQ & PIRQ
From: "pradeep singh rautela" <rautelap@xxxxxxxxx>
Date: Wed, 6 Jun 2007 13:49:27 +0530
Cc: "Gautham Kampalapur Shankar, TLS, Chennai" <gauthamk@xxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxx
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Hi Mark,
[snip]

If I recall correctly, PIRQs are mapped one-to-one onto the lower numbered
event channels (I'm not sure if this is specific to XenLinux or part of the
Xen API).

A VIRQ is a interrupt-like event that originates from Xen itself.  An example
would be notifications from the Xen virtual timer device.  Again, the guest
sees it as an event channel notification but this time we call it a VIRQ to
show that it comes from an entirely *virtual* source.
Does this means if hypervisor recieves an interrupt from an external
card on the behalf of a domU, it actually buffers it like a an APIC
before eventually generating a VIRQ to the respective domU?

The above question is more obvious when we have an external hardware
device which generates interrupts are a very high rate e.g a busy
network card.

So, does Xen buffers the recieved IRQs if it is unable to send the IRQ
to the correct domU?

I hope same holds true while handling interrupts on the behalf of dom0.
Or is it that, Xen just forwards the interrupt to dom0 which then
handles the actual creation of a VIRQ using event channels to notify a
domU?

Thank you
- --psr


There are actually more variations to event channels: events might also come
from another domain we are communicating with, or (I think) from another VCPU
in the same domain.

Hope this helps,
Cheers,
Mark

--
Dave: Just a question. What use is a unicyle with no seat?  And no pedals!
Mark: To answer a question with a question: What use is a skateboard?
Dave: Skateboards have wheels.
Mark: My wheel has a wheel!

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--
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pradeep singh rautela

"Genius is 1% inspiration, and 99% perspiration" - not me :)

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