netdev - Re: [RFC PATCH 00/17] virtual-bus

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Date:	Thu, 02 Apr 2009 14:48:11 +0300
From:	Avi Kivity <avi@...hat.com>
To:	Gregory Haskins <ghaskins@...ell.com>
CC:	Herbert Xu <herbert@...dor.apana.org.au>, anthony@...emonkey.ws,
	andi@...stfloor.org, linux-kernel@...r.kernel.org, agraf@...e.de,
	pmullaney@...ell.com, pmorreale@...ell.com, rusty@...tcorp.com.au,
	netdev@...r.kernel.org, kvm@...r.kernel.org
Subject: Re: [RFC PATCH 00/17] virtual-bus

Gregory Haskins wrote:

  

>> There is no choice.  Exiting from the guest to the kernel to userspace
>> is prohibitively expensive, you can't do that on every packet.
>>
>>     
>
> Now you are making my point ;)  This is part of the cost of your
> signaling path, and it directly adds to your latency time.   

It adds a microsecond.  The kvm overhead of putting things in userspace 
is low enough, I don't know why people keep mentioning it.  The problem 
is the kernel/user networking interfaces.

> You can't
> buffer packets here if the guest is only going to send one and wait for
> a response and expect that to perform well.  And this is precisely what
> drove me to look at avoiding going back to userspace in the first place.
>   

We're not buffering any packets.  What we lack is a way to tell the 
guest that we're done processing all packets in the ring (IOW, re-enable 
notifications).

-- 
error compiling committee.c: too many arguments to function

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