Re: [Qemu-devel] [PATCH 2/2] Route IOAPIC interrupts via ISA bus

[Beth Kon <eak@us.ibm.com>]

Avi Kivity wrote:
> On 08/28/2009 05:20 AM, Beth Kon wrote:
>> I was thinking of the HPET advertising one or more of these new 
>> ioapic interrupts so that it could support non-legacy operation. It 
>> can't at the moment because there are no interrupt lines available. I 
>> haven't looked into the details of what would remain to be done to 
>> make this happen, but does it sound reasonable? Sounds like at a 
>> minimum acpi needs some work.
>
> What does non-legacy HPET buy us?
>
Actually, it looks like not much. I was thinking about users of 
/dev/hpet, but there really don't seem to be any. I found posts talking 
about deprecating the hpet ioctl interface due to availability of POSIX 
timers, so it doesn't look like anything is on the horizon. Right now, 
linux has no HPET requirements that aren't handled by timers 0 and 1 
that are currently implemented for qemu/kvm, and as far as I can tell, 
Windows has no such need either.

But, to be precise, even if the HPET runs in legacy mode, the standard 
hardware implementation as I understand it has a third timer available 
that is not involved with legacy operation. Because there were no spare 
interrupts around, I recently submitted patches to remove that third 
timer (that doesn't appear to be used anywhere). When I saw the possible 
availability of more APIC interrupt lines, I thought about using that 
for  at least this third timer.

The above thoughts came about after Andriy Gapon brought up some issues 
with available HPET interrupts. Andriy, do you have anything to add here 
about potential use of a third timer or non-legacy mode? Right now I'm 
inclined to think there isn't much use, and limiting HPET to legacy mode 
would be fine.