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

[Anthony Liguori <anthony@codemonkey.ws>]

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<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.
>>
>
> 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.

Regards,

Anthony Liguori

>