Subject: Re: [PATCHv5 3/3] vhost_net: a kernel-level virtio server

From: Gregory Haskins <[email-masked]>
Date: Tue, 15 Sep 2009 09:50:39 -0400

Avi Kivity wrote:
> On 09/15/2009 04:03 PM, Gregory Haskins wrote:
>>
>>> In this case the x86 is the owner and the ppc boards use translated
>>> access.  Just switch drivers and device and it falls into place.
>>>
>>>      
>> You could switch vbus roles as well, I suppose.
> 
> Right, there's not real difference in this regard.
> 
>> Another potential
>> option is that he can stop mapping host memory on the guest so that it
>> follows the more traditional model.  As a bus-master device, the ppc
>> boards should have access to any host memory at least in the GFP_DMA
>> range, which would include all relevant pointers here.
>>
>> I digress:  I was primarily addressing the concern that Ira would need
>> to manage the "host" side of the link using hvas mapped from userspace
>> (even if host side is the ppc boards).  vbus abstracts that access so as
>> to allow something other than userspace/hva mappings.  OTOH, having each
>> ppc board run a userspace app to do the mapping on its behalf and feed
>> it to vhost is probably not a huge deal either.  Where vhost might
>> really fall apart is when any assumptions about pageable memory occur,
>> if any.
>>    
> 
> Why?  vhost will call get_user_pages() or copy_*_user() which ought to
> do the right thing.

I was speaking generally, not specifically to Ira's architecture.  What
I mean is that vbus was designed to work without assuming that the
memory is pageable.  There are environments in which the host is not
capable of mapping hvas/*page, but the memctx->copy_to/copy_from
paradigm could still work (think rdma, for instance).

> 
>> As an aside: a bigger issue is that, iiuc, Ira wants more than a single
>> ethernet channel in his design (multiple ethernets, consoles, etc).  A
>> vhost solution in this environment is incomplete.
>>    
> 
> Why?  Instantiate as many vhost-nets as needed.

a) what about non-ethernets?
b) what do you suppose this protocol to aggregate the connections would
look like? (hint: this is what a vbus-connector does).
c) how do you manage the configuration, especially on a per-board basis?

> 
>> Note that Ira's architecture highlights that vbus's explicit management
>> interface is more valuable here than it is in KVM, since KVM already has
>> its own management interface via QEMU.
>>    
> 
> vhost-net and vbus both need management, vhost-net via ioctls and vbus
> via configfs.

Actually I have patches queued to allow vbus to be managed via ioctls as
well, per your feedback (and it solves the permissions/lifetime
critisims in alacrityvm-v0.1).

>  The only difference is the implementation.  vhost-net
> leaves much more to userspace, that's the main difference.

Also,

*) vhost is virtio-net specific, whereas vbus is a more generic device
model where thing like virtio-net or venet ride on top.

*) vhost is only designed to work with environments that look very
similar to a KVM guest (slot/hva translatable).  vbus can bridge various
environments by abstracting the key components (such as memory access).

*) vhost requires an active userspace management daemon, whereas vbus
can be driven by transient components, like scripts (ala udev)

Kind Regards,
-Greg