Re: [patch] x86_64, irq: check remote IRR bit before migrating level triggered irq

[ebiederm@xmission.com (Eric W. Biederman)]

"Siddha, Suresh B" <suresh.b.siddha@intel.com> writes:

> Eric,
>
> On Fri, May 18, 2007 at 07:40:53AM -0700, Eric W. Biederman wrote:
>> Still in any of those I don't see a problem with switching to edge
>> triggered mode and then back again.  Either Remote IRR will keep
>> it's current state or it will be cleared.   Remote IRR should not
>> get set (when it was clear) by such a procedure because that
>> would mess up the normal interrupt enable sequence that happens
>> on boot.  So I'm pretty certain toggling the edge bit is harmless
>> and it may actually clear Remote IRR for us.
> ...
>> 
>> I think going more the way that this code has gone on arch/i386 with
>> real functions is preferable.
>
> There is an issue with this suggestion. We have an inflight
> EOI broadcast msg to this IOAPIC (that got delayed but still alive) and
> that can cause problem if we use edge and back to level to reset remote IRR bit.
>
> If the vector number stays same during irq migration and if we reset remote
> IRR bit using the above method(edge and then back to level) during
> irq migration, then we have a problem. A new interrupt arriving on a new
> cpu will set the remote IRR bit and now the old inflight EOI broadcast
> reaches IOAPIC RTE(resetting the remote IRR bit, because the vector in the
> broadcast msg is same), while the kernel code still assumes that the remote
> IRR bit is still set. This will lead to more problems and issues.

Ok.  I will agree that if the level->edge->level switch does not reset
Remote_IRR and the EOI arrives in that window.  We have another condition
in which Remote_IRR can get stuck on.

Eric