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

From: Avi Kivity <[email-masked]>
Date: Thu, 02 Apr 2009 14:43:39 +0300

Gregory Haskins wrote:

>> virtio is already non-kvm-specific (lguest uses it) and
>> non-pci-specific (s390 uses it).
>>     
>
> Ok, then to be more specific, I need it to be more generic than it
> already is.  For instance, I need it to be able to integrate with
> shm_signals.  

Why?

>> If you have a good exit mitigation scheme you can cut exits by a
>> factor of 100; so the userspace exit costs are cut by the same
>> factor.  If you have good copyless networking APIs you can cut the
>> cost of copies to zero (well, to the cost of get_user_pages_fast(),
>> but a kernel solution needs that too).
>>     
>
> "exit mitigation' schemes are for bandwidth, not latency.  For latency
> it all comes down to how fast you can signal in both directions.  If
> someone is going to do a stand-alone request-reply, its generally always
> going to be at least one hypercall and one rx-interrupt.  So your speed
> will be governed by your signal path, not your buffer bandwidth.
>   

The userspace path is longer by 2 microseconds (for two additional 
heavyweight exits) and a few syscalls.  I don't think that's worthy of 
putting all the code in the kernel.

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