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

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2011-08-31 19:41, Blue Swirl wrote:
> On Wed, Aug 31, 2011 at 10:53 AM, Jan Kiszka <jan.kiszka@siemens.com> 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)
> 
> Why, only for b)? This is not possible with real hardware.
> 
>> 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.
> 
> I think this would need pretty heavy changes everywhere. At board
> level the full path needs to be identified and special versions of
> IRQs installed along the way. The routers would need to use callbacks
> to inform other parties about routing changes.

It already works in practice (based on a hack and minus IRQ router state
updates) for x86 PCI device pass-through. At least I don't want this
upstream but instead a generic solution. The ability to skip IRQ routers
also in pure user space device model scenarios is a useful by-product.

> 
>> 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.
> 
> Perhaps a global IRQ manager could help. It would keep track of the
> whole IRQ matrix, what are input (x axis) and output (y axis) states
> and what each matrix node (router state) looks like (or able to
> compute) if asked. I don't think backward links would be needed with
> this approach.

Well, the backward links would then be moved to that global IRQ manager.
It's just moving the data management, but if it turns out to allow a
cleaner device design, I would surely not vote against it. But that
manager must support lazy updates as well because we cannot call it from
kernel space for each and every event.

Jan

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