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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 08:32:54 -0500
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On 09/04/2011 07:13 AM, Jan Kiszka wrote:
On 2011-09-03 21:54, Anthony Liguori wrote:
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.

INTx support for device assignment, vhost, or any other future in-kernel
IRQ sources.

I prefer to not think of IRQs as special things. They're just single bits of information that flow through the device model. Having a higher level representation that understands something like paths seems wrong to me.

I'd prefer to treat things like device assignment as a hack. You just need code that can walk the device tree to figure out the path (which is not generic code, it's very machine specific). Then you tell the kernel the resolution of the path and are otherwise completely oblivious in userspace.


Anthony Liguori

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