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

From: Jan Kiszka
Subject: Re: [Qemu-devel] [PATCH] pc: Clean up PIC-to-APIC IRQ path
Date: Sun, 04 Sep 2011 14:37:17 +0200
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On 2011-09-04 14:17, 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
>> 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.
> 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).

With current kvm device assignment it's mandatory as it only support
kernel/kernel IRQ delivery. Only vfio's eventfds will make it optional
(but still highly desirable).


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