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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 08:35:30 -0500
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On 09/04/2011 07:17 AM, Avi Kivity wrote:
On 08/31/2011 01:53 PM, Jan Kiszka wrote:
On 2011-08-31 10:25, Peter Maydell wrote:
> On 30 August 2011 20:28, Jan Kiszka<address@hidden> wrote:
>> Yes, that's the current state. Once we have bidirectional IRQ links in
>> place (pushing downward, querying upward - required to skip IRQ
>> for fast, lockless deliveries), that should change again.
> Can you elaborate a bit more on this? I don't think anybody has
> proposed links with their own internal state before in the qdev/qom
> discussions...

That basic idea is to allow

a) a discovery of the currently active IRQ path from source to sink
(that would be possible via QOM just using forward links)

b) skip updating the states of IRQ routers in the common case, just
signaling directly the sink from the source (to allow in-kernel IRQ
delivery or to skip taking some device locks). Whenever some router
is queried for its current IRQ line state, it would have to ask the
preceding IRQ source for its state. So we need a backward link.

We haven't thought about how this could be implemented in details yet
though. Among other things, it heavily depends on the final QOM design.

Looks like a similar path to the memory API. A declarative description
of the interrupt hierarchy allows routes to be precalculated and flattened.

(here it's strictly an optimization; with the memory API it's a
requirement since kvm requires a flattened representation, and tcg is
greatly simplified by it).

Let's not be careful to over generalize here. Memory access is a fairly complicated thing involving multiple different types of busses, various properties (endianness, alignment, access constraints). IRQs are extremely simple. They're just single bits of information.


Anthony Liguori

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