[Xen-ia64-devel] Event channel vs current scheme speed [was vIOSAPIC and IRQs delivery]

[Tristan Gingold <Tristan.Gingold@xxxxxxxx>]

I'd like to narrow the discussion on this problem within this thread.

We all agree we'd like to support shared IRQs and drivers domain.
I am saying I can do that without event channel, while Eddie says it is 
required.

One of Eddie argument is performance, as discussed previously.  Since I don't 
agree and things should be obvious here, I'd like to understand why we don't 
agree.

See my comments.

> >> 4: More new hypercall is introduced and more call to hypervisor.
> > Only physdev_op hypercall is added, but it is also used in x86 to set
> > up IOAPIC.  You can't avoid it.
>
> Initial time is OK for no matter what approach, runtime is critical.
Ok.

> I saw a lot of hypercall for RTE write.
Did you see them by reading the code or by running ?

There are hypercalls to mask/unmask interrupts.  Is it a performance 
bottleneck ?  I don't think so, since masking/unmasking shouldn't be very 
frequent.  Please tell me if I am wrong.

There are also hypercalls to do EOI.  This can be a performance issue.
If the interrupt is edge triggered, EOI is not required and could be optimized 
out if not yet done.
If the interrupt is level-triggered, then it is required and I don't 
understand how event-channel can avoid it.  For me, this is the purpose of 
hypercall PHYSDEVOP_IRQ_UNMASK_NOTIFY.  Xen has to know when all domains have 
finished to handle the interrupt.

Finally, there is the LSAPIC TPR, IVR and EOI.  I think the overhead is very 
small thanks to Dan's work.  And this overhead was measured with the timer.

In my current understanding, I don't see the performance gain of 
event-channel.  And I repeat, I'd like to understand.

Tristan.


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