IRQ: issues with directed EOI and IO-APIC ack methods

[Andrew Cooper <andrew.cooper3@citrix.com>]

Hello,

XenServer6.0 (Xen 4.1.1) has had a support escalation against it for
Cisco C210 M2 servers.  I do not have access to any of these servers, so
cant debug the issue myself.

The pcpu LAPICs support EOI Broadcast suppression and Xen enabled it. 
In arch/x86/apic.c:verify_local_APIC, there is a comment stating that
directed EOI support must use the old IO-APIC ack method.

A hypervisor with this check disabled (i.e. never checking for, or
enabling directed EOI) seems to make the system stable again (5 days
stable now, as opposed to a hang due to lost interrupts once every few
hours before).

First of all, I have discovered that forcing "ioapic_ack=new" does not
have the indented effect, because verify_local_APIC trashes it, even if
the user has specified the ack method.  I intend to send a patch to fix
this in due course.

However, as for the main issue, I cant work out any logical reason why
directed EOI would not work with the new ack mode.  I am still trying to
work out the differences in the code path incase I have missed something
subtle, but I wondered if anyone on the list has more knowledge of these
intricacies than me?  Either way, it appears that there is a bug on the
codepath with directed EOI and old ack method.

Thanks in advance,

-- 
Andrew Cooper - Dom0 Kernel Engineer, Citrix XenServer
T: +44 (0)1223 225 900, http://www.citrix.com