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Date: Wed, 31 Aug 2011 12:53:11 +0200
From: Jan Kiszka <jan.kiszka@siemens.com>
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Subject: Re: [Qemu-devel] [PATCH] pc: Clean up PIC-to-APIC IRQ path
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: Lucas Meneghel Rodrigues <lmr@redhat.com>, Anthony Liguori <aliguori@us.ibm.com>, Marcelo Tosatti <mtosatti@redhat.com>, qemu-devel <qemu-devel@nongnu.org>, Blue Swirl <blauwirbel@gmail.com>, Avi Kivity <avi@redhat.com>, Gerd Hoffmann <kraxel@redhat.com>

On 2011-08-31 10:25, Peter Maydell wrote:
> On 30 August 2011 20:28, Jan Kiszka <jan.kiszka@web.de> 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
> 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.

Jan

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