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

[Avi Kivity]

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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