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Date: Wed, 13 May 2009 06:53:34 -0400
From: Gregory Haskins <ghaskins@novell.com>
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To: Anthony Liguori <anthony@codemonkey.ws>
CC: Gregory Haskins <gregory.haskins@gmail.com>, Avi Kivity <avi@redhat.com>,
       Chris Wright <chrisw@sous-sol.org>, linux-kernel@vger.kernel.org,
       kvm@vger.kernel.org, Hollis Blanchard <hollisb@us.ibm.com>
Subject: Re: [RFC PATCH 0/3] generic hypercall support
References: <20090505132005.19891.78436.stgit@dev.haskins.net> <4A0040C0.1080102@redhat.com> <4A0041BA.6060106@novell.com> <4A004676.4050604@redhat.com> <4A0049CD.3080003@gmail.com> <20090505231718.GT3036@sequoia.sous-sol.org> <4A010927.6020207@novell.com> <20090506072212.GV3036@sequoia.sous-sol.org> <4A018DF2.6010301@novell.com> <4A02D40D.7060307@redhat.com> <4A0448DF.90705@codemonkey.ws> <4A0570B1.30401@novell.com> <4A071F1A.1090702@codemonkey.ws> <4A0824C2.4000109@gmail.com> <4A0860D7.6010708@codemonkey.ws>
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Anthony Liguori wrote:
> Gregory Haskins wrote:
>>
>> So, yes, the delta from PIO to HC is 350ns.  Yes, this is a ~1.4%
>> improvement.  So what?  Its still an improvement.  If that improvement=

>> were for free, would you object?  And we all know that this change isn=
't
>> "free" because we have to change some code (+128/-0, to be exact).  Bu=
t
>> what is it specifically you are objecting to in the first place?  Addi=
ng
>> hypercall support as an pv_ops primitive isn't exactly hard or complex=
,
>> or even very much code.
>>  =20
>
> Where does 25us come from?  The number you post below are 33us and 66us=
=2E

<snip>

The 25us is approximately the max from an in-kernel harness strapped
directly to the driver gathered informally during testing.  The 33us is
from formally averaging multiple runs of a userspace socket app in
preparation for publishing.  I consider the 25us the "target goal" since
there is obviously overhead that a socket application deals with that
theoretically a guest bypasses with the tap-device.  Note that the
socket application itself often sees < 30us itself...this was just a
particularly "slow" set of runs that day.

Note that this is why I express the impact as "approximately" (e.g.
"~4%").  Sorry for the confusion.

-Greg


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