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

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Message-ID: <CAEDV+g+ard7e86vmfo6XF+1AmkndYVqnv_nRS3fXEx317MuXaA@mail.gmail.com>
Date:	Sun, 7 Apr 2013 14:25:19 -0700
From:	Christoffer Dall <cdall@...columbia.edu>
To:	"Michael S. Tsirkin" <mst@...hat.com>
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 Sun, Apr 7, 2013 at 12:41 AM, Michael S. Tsirkin <mst@...hat.com> wrote:
> 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.
>

the implementation effort to simply measure the hypercall performance
should be minimal, no? If we can measure a true difference in
performance, I'm sure we can revisit the issue of what will be merged
and what won't be, but until we have those numbers it's all
speculation.

>> 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.)
>>
>
> 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?
>
We have a branch called kvm-selftest that replicates much of the
functionality, which is what I run to get these measurements. I can
port it over to unittest at some point, but I'm not active working on
that.

I can measure it, but we have bigger fish to fry on the ARM side right
now, so it'll be a while until I get to that.
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