linux-kernel - Re: [RFC PATCH 0/3] generic hypercall support

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Date:	Wed, 13 May 2009 06:53:34 -0400
From:	Gregory Haskins <ghaskins@...ell.com>
To:	Anthony Liguori <anthony@...emonkey.ws>
CC:	Gregory Haskins <gregory.haskins@...il.com>,
	Avi Kivity <avi@...hat.com>,
	Chris Wright <chrisw@...s-sol.org>,
	linux-kernel@...r.kernel.org, kvm@...r.kernel.org,
	Hollis Blanchard <hollisb@...ibm.com>
Subject: Re: [RFC PATCH 0/3] generic hypercall support

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).  But
>> what is it specifically you are objecting to in the first place?  Adding
>> hypercall support as an pv_ops primitive isn't exactly hard or complex,
>> or even very much code.
>>   
>
> Where does 25us come from?  The number you post below are 33us and 66us.

<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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