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Re: [Qemu-devel] Re: [PATCH 0/4] pci: interrupt status/interrupt disable

From: Michael S. Tsirkin
Subject: Re: [Qemu-devel] Re: [PATCH 0/4] pci: interrupt status/interrupt disable support
Date: Thu, 26 Nov 2009 15:34:09 +0200
User-agent: Mutt/1.5.19 (2009-01-05)

On Thu, Nov 26, 2009 at 01:21:39PM +0000, Paul Brook wrote:
> >> It's really not that much of a fast path. Unless you're doing something
> >> particularly obscure then even under heavy load you're unlikely to exceed
> >> a few kHz.
> >
> >I think with kvm, heavy disk stressing benchmark can get higher.
> I'd still expect this to be the least of your problems. 
> If nothing else you've at least one host signal delivery and/or thread 
> context 
> switch in there.

iotread which does the signalling might be running in parallel
with the guest CPU.

> Not to mention the overhead to forwarding the interrupt to 
> the guest CPU.

This is often mitigated as KVM knows to inject the interrupt
on the next vmexit.

> > > Compared to the average PIC implementation, and the overhead of the
> > > actual CPU interrupt, I find it hard to believe that looping over
> > > precisely 4 entries has any real performance hit.
> > 
> > I don't think it is major, but I definitely have seen, in the past,
> > that extra branches and memory accesses have small but measureable effect
> > when taken in interrupt handler routines in drivers, and same should
> > apply here.
> > 
> > OTOH keeping the sum around is trivial.
> Not entirely. You now have two different bits of information that you have to 
> keep consistent.

This is inherent in pci spec definition: interrupt status
bit in config space duplicates interrupt state.

> Unless you can show that this is performance critical code I strongly 
> recommend keeping it as simple as possible.
> Paul

I don't see there is anything left show: interrupt delivery is *obviously*
performance critical: people are running *latency benchmarks* measuring
how fast a packet can get from an external interface into guest,
in microseconds. We definitely want to remove obvious waste there.


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