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Re: xvif(4) and kernel.org domU

On Sun, Jun 29, 2008 at 12:57:12PM +0200, Manuel Bouyer wrote:
> On Sun, Jun 29, 2008 at 05:00:55AM +0000, Jonathan A. Kollasch wrote:
> > Hi,
> > 
> > When you boot a kernel.org x86_32p domU
> > on a NetBSD/amd64 dom0, you get this lovely message:
> > 
> > net eth0: backend does not support copying receive path
> > 
> > This is why eth0 then shows up with a null MAC address
> > and doesn't work.
> > 
> > Anyway, our network backend driver can only do "flipping".
> > The kernel.org network frontend can only handle "copying".
> Do you know what the difference is between "flipping" and "copying" ?

Well, to paraphrase a book excerpt:

Flipping is where ownership of pages changes between the two domains.
Copying is when pages stay owned by their respective domains,
and the hypervisor copies the data between the domain's pages.

It is said that flipping caused TLB churn which decreased performance.

        Jonathan Kollasch

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