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Re: [Qemu-devel] [PATCH 11/22] i8259: Update IRQ state after reset

From: Jan Kiszka
Subject: Re: [Qemu-devel] [PATCH 11/22] i8259: Update IRQ state after reset
Date: Fri, 30 Sep 2011 08:47:28 +0200
User-agent: Mozilla/5.0 (X11; U; Linux i686 (x86_64); de; rv: Gecko/20080226 SUSE/ Thunderbird/ Mnenhy/

On 2011-09-29 21:45, Blue Swirl wrote:
> On Wed, Sep 28, 2011 at 9:18 PM, Jan Kiszka <address@hidden> wrote:
>> On 2011-09-28 20:01, Blue Swirl wrote:
>>> On Wed, Sep 28, 2011 at 11:00 AM, Jan Kiszka<address@hidden>
>>>  wrote:
>>>> As we clearly modify the PIC state on pic_reset, we also have to update
>>>> the IRQ output. This only happened on init so far. Apply this
>>>> consistently.
>>> Nack, IRQ lines shouldn't be touched on reset. The other side may not
>>> be ready for receiving the interrupt change and qemu_irqs are
>>> stateless anyway.
>> Sorry, but failing to clear the line (this is what pic_update_irq will
>> effectively do) is a clear bug in the current code. This patch is 100%
>> analogue to what, e.g. the PCI layer does on reset. Please re-read.
> Reset will happen also when the devices are created. At that time,
> qemu_irq callback triggered by changing of the state may produce
> undesired effects on the other side.

All those potential effects will be cleared again when the receiver is
reset as well. If not, that would be a bug which requires fixing.

> There have been bugs earlier, see
> bc26e55a6615dc594be425d293db40d5cdcdb84b and
> 42f1ced228c9b616cfa2b69846025271618e4ef5 and discussion in
> http://lists.nongnu.org/archive/html/qemu-devel/2009-06/msg01024.html.

Deasserting IRQs at PCI device level is indeed useless as we already do
this in the PCI core.

There is no difference between system reset after power-up or later on.
And there should be no difference between per device, per group of
devices or system-wide reset. A device model cannot tell these apart.
That the devices reset handler is only called on system reset is an odd
and fragile assumption - not that fragile for platform devices like the
i8259, but definitely bogus for pluggable ones.

My series does not depend on this cleanup/fix, but the reason not to
apply remains wrong IMHO.


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