Re: [Qemu-devel] [RFT][PATCH 07/15] qemu_irq: Add IRQ handlers with delivery feedback

[Jan Kiszka]

Gleb Natapov wrote:
> On Mon, May 24, 2010 at 10:13:40PM +0200, Jan Kiszka wrote:
>> From: Jan Kiszka <address@hidden>
>>
>> This allows to communicate potential IRQ coalescing during delivery from
>> the sink back to the source. Targets that support IRQ coalescing
>> workarounds need to register handlers that return the appropriate
>> QEMU_IRQ_* code, and they have to propergate the code across all IRQ
>> redirections. If the IRQ source receives a QEMU_IRQ_COALESCED, it can
>> apply its workaround. If multiple sinks exist, the source may only
>> consider an IRQ coalesced if all other sinks either report
>> QEMU_IRQ_COALESCED as well or QEMU_IRQ_MASKED.
>>
> Well, almost two years passed since this approach was proposed first
> time[1] ;). Back then it generated bunch of nonsensical comments about
> real hardware not working this way, so the hack that we have now was
> introduce to overcome this resistance. I hope enough time passed for
> people to gain some sense and the approach will be adopted this time.
> Really this should have been done two year ago.
> 
> [1] http://lists.nongnu.org/archive/html/qemu-devel/2008-06/msg00757.html

Yeah, I somehow had a vague feeling that there must have been an earlier
attempt. I think my approach could be a slightly easier to accept as it
does not require converting all platforms, but this can happen on demand
(or not at all). Moreover, I think that the third return state,
QEMU_IRQ_MASKED, is important for correct handling of multiple IRQ sinks.

However, as I would see it now, we just have two options long term:
 - drop IRQ coalescing workarounds
 - properly support them via qemu_irq

The current hack cannot stay. E.g., it does not scale because it depends
on a global variable of the APIC. So we would never able to protect the
APICs with individual locks.

Jan

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