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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: Sat, 03 Sep 2011 14:54:47 -0500
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On 08/31/2011 05:53 AM, 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 routers
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

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.

Can you provide some concrete use-cases of this? I'm not convinced this is really all that important and it seems like tremendous amounts of ugliness would be needed to support it.


Anthony Liguori

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.


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