Re: [Xen-devel] [PATCH 0/4] Support accelerated network plugin modules

[Kieran Mansley <kmansley@xxxxxxxxxxxxxx>]

On Thu, 2007-05-31 at 22:04 +0530, Jambunathan K wrote:
> I have a question regarding the feature set that is proposed here. The
> intention is to further my own understanding and get some additional
> insights.
> 
> Doesn't accelerated network plugins solve the same problem as say a
> "native" PCI passthroughed driver. Put other way, what can an
> accelerator do that a passthroughed driver cannot do?

Functionally they're similar, in that they both enable a guest to get
direct access to the NIC to deliver higher network performance, but off
the top of my head:

 - PCI passthrough would require hardware to support multiple PCI
devices.  The accelerated plugin approach we've put forward doesn't (but
would also work fine with devices that virtualise at the PCI layer), and
so would support a wider range of devices.

 - You're limited with PCI passthrough to the number of PCI devices your
NIC can support, which is (for the NICs I've seen) considerably less
than the number of VIs that other NICs can support.  This will limit the
number of guests you can provide accelerated network interfaces to.

 - I'm not sure what the story is for migration with PCI passthrough,
but with the accelerated plugin approach your network interfaces can
transparently (as far as the guest's network stack is concerned) move
from being accelerated to not-accelerated (and vice-versa) depending on
what hardware is available, and migrate from one type of accelerated
hardware to another.  

 - A PCI-passthrough-based network interface might struggle to route
packets to a virtual machine on the same host.  The accelerated network
plugin can just fall back to the netfront/netback normal network path to
get packets to these, or use facilities on the NIC to achieve the same
if available.

In summary, it's "passthrough" rather than "PCI passthrough", if that
makes sense.

Kieran


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