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Re: [Qemu-devel] [PATCH] pc: Clean up PIC-to-APIC IRQ path

From: Anthony Liguori
Subject: Re: [Qemu-devel] [PATCH] pc: Clean up PIC-to-APIC IRQ path
Date: Sun, 04 Sep 2011 09:43:51 -0500
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On 09/04/2011 09:12 AM, Avi Kivity wrote:
On 09/04/2011 04:41 PM, Anthony Liguori wrote:
See it as you like, but we need the support, not only for device
assigment. And I do not see any gain it hacking this instead of
designing it.

You can design a hack but it's still a hack.

Device state belongs in devices. Trying to extract device state
(interrupt routing paths) and externalizing it is by definition a hack.

Pet peeve - saying something is "by definition" a hack is just rhetoric
unless the definition of device state is "something that cannot be
extracted and externalized". Let's avoid this.

Likewise, I would prefer to avoid stating that something is a hard requirement in the absence of concrete use-cases.

I do think the definition of device state is more or less something that is opaque and owned by the device. The more we externalize the device state, the more we have to deal with compatibility (even if it's just internal compatibility). This makes modularity hard because there's too many things with complex dependencies.

In fact it's exactly what we do with the memory API. Memory routing is
part of device state, yet we expose it to the memory API and let it do
its thing instead of going through the hierarchy on every single memory

Yes, and the memory API is complicated and invasive :-) But it's worth it at the end of the day (although I think it could be simplified at the expensive of not allowing as much flattening).

Whether it's worthwhile to do this for interrupts as well depends on how
common the situation is and what we gain from it. We can only do this if
the routing is semi-static (depends only on other state but not the
actual interrupt) - probably the only exception to this is the "least
priority" mode which means the last leg cannot be computed statically.

I agree the gain is low if we limit ourselves to 440fx, but if we add
interrupt remapping it becomes considerably more complicated to do this
backward path computation.

If we start with a simple interface that works for the i440fx and then expand it as we support more layers, I think we'll be okay.

What I'm concerned about is an attempt to globally track IRQ routing. I can imagine constructing a board where you have two simple devices that have level triggered interrupts and wanting to tie them to a single input pin. A simple OR gate would be sufficient to do this. Having to make an OR gate an IRQ router seems like far too much complexity to me.

I think we need to strive to keep simple things simple.


Anthony Liguori

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