Re: PCI pass-through of NIC to PV DomU... worth it?

[Manuel Bouyer <bouyer%antioche.eu.org@localhost>]

On Fri, Feb 24, 2012 at 02:49:21PM +0530, Cherry G. Mathew wrote:
> Hi David,
> 
> On 24 February 2012 09:21, David Howland <metalliqaz%fastmail.fm@localhost> 
> wrote:
> > Either my Google-fu is weak or nobody has actually asked this in the open
> > before...
> >
> > Is it worth it to try and use PCI pass-through to give a NIC to a PV DomU?
> >  I'm setting up a DomU firewall/router.  I want the latency to be as low as
> > possible.
> >
> > I need to know which is faster: the PV network interface or passing the NIC?
> 
> That would depend on your card - if it's got "hardware acceleration"
> (TSO, et. al) it could free up the main cpu. Also note that a single
> backend driver handles *all* your domU traffic, so this is a potential
> bottleneck (I have no idea if it is in practice).
> 
> Why not try both and find out ?
> 
> >  I don't have VT-d support, but the server is a dual quad-core Xeon setup,
> > so I shouldn't be short on compute cycles.  My gut is telling me that there
> > wouldn't be any tangible difference, but I want to hear what the experts
> > think.
> 
> I'm not an expert, and I haven't tried it, but I think PCI passthrough
> on xen requires VT-d (for partitioning/security rather than
> efficiency).

No, it does not. And indeed, the domU can compromise the whole machine
when loaned a DMA-capable PCI device.

-- 
Manuel Bouyer <bouyer%antioche.eu.org@localhost>
     NetBSD: 26 ans d'experience feront toujours la difference
--