linux-kernel - Re: [PATCH RFC] kvm: add PV MMIO EVENTFD

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Date:	Sun, 7 Apr 2013 10:41:13 +0300
From:	"Michael S. Tsirkin" <mst@...hat.com>
To:	Christoffer Dall <cdall@...columbia.edu>
Cc:	Alexander Graf <agraf@...e.de>, Gleb Natapov <gleb@...hat.com>,
	Marcelo Tosatti <mtosatti@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Xiao Guangrong <xiaoguangrong@...ux.vnet.ibm.com>,
	Takuya Yoshikawa <yoshikawa_takuya_b1@....ntt.co.jp>,
	Alex Williamson <alex.williamson@...hat.com>,
	Will Deacon <will.deacon@....com>,
	Sasha Levin <sasha.levin@...cle.com>,
	Andrew Morton <akpm@...ux-foundation.org>, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	virtualization@...ts.linux-foundation.org
Subject: Re: [PATCH RFC] kvm: add PV MMIO EVENTFD

On Thu, Apr 04, 2013 at 04:32:01PM -0700, Christoffer Dall wrote:
> [...]
> 
> >> >> to give us some idea how much performance we would gain from each approach? Thoughput should be completely unaffected anyway, since virtio just coalesces kicks internally.
> >> >
> >> > Latency is dominated by the scheduling latency.
> >> > This means virtio-net is not the best benchmark.
> >>
> >> So what is a good benchmark?
> >
> > E.g. ping pong stress will do but need to look at CPU utilization,
> > that's what is affected, not latency.
> >
> >> Is there any difference in speed at all? I strongly doubt it. One of virtio's main points is to reduce the number of kicks.
> >
> > For this stage of the project I think microbenchmarks are more appropriate.
> > Doubling the price of exit is likely to be measureable. 30 cycles likely
> > not ...
> >
> I don't quite understand this point here. If we don't have anything
> real-world where we can measure a decent difference, then why are we
> doing this? I would agree with Alex that the three test scenarios
> proposed by him should be tried out before adding this complexity,
> measured in CPU utilization or latency as you wish.

Sure, plan to do real world benchmarks for PV MMIO versus PIO as well.
I don't see why I should bother implementing hypercalls given that the
kvm maintainer says they won't be merged.

> FWIW, ARM always uses MMIO and provides hardware decoding of all sane
> (not user register-writeback) instruction, but the hypercall vs. mmio
> looks like this:
> 
> hvc:  4,917
> mmio_kernel: 6,248

So 20% difference?  That's not far from what happens on my intel laptop:
vmcall 1519
outl_to_kernel 1745
10% difference here.

> 
> But I doubt that an hvc wrapper around mmio decoding would take care
> of all this difference, because the mmio operation needs to do other
> work not realated to emulating the instruction in software, which
> you'd have to do for an hvc anyway (populate kvm_mmio structure etc.)
> 
> -Christoffer

Instead of speculating, someone with relevant hardware
could just try this, but kvm unittest doesn't seem to have arm support
at the moment. Anyone working on this?

-- 
MST
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