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

From: Avi Kivity <[email-masked]>
Date: Thu, 02 Apr 2009 14:48:11 +0300

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).

-- 
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