Port-xen archive

[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index][Old Index]

Re: xvif(4) and kernel.org domU

Manuel Bouyer wrote:
On Sun, Jun 29, 2008 at 01:56:16PM +0000, Jonathan A. Kollasch wrote:
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.
Hum, I guess this is a feature that appeared in Xen 3.2. I don't remember
seeing anything like that in Xen 3.1.x
IIRC, copying vs flipping appeared with xen 3.X, and frontend queries backend about support for copy or flipping through xenbus (see feature-rx-copy in xenstore). When the string is absent, it defaults to page flip (like 2.6.18 on xensource does). Since netbsd does not add feature-rx-* variables to xenstore, it defaults to page flipping for a domU.

Downside is, linux vanilla removed the support for flipping about approx. a year ago, making copy the only way to plug back end and front end together for net device.

(BTW, it made the xen compatibility between linux distros a real pain, as some used to use flipping for netfront driver).

3.2 requires copying anyway, so that will be mandatory for netbsd when 3.2 support is added.


Jean-Yves Migeon

Home | Main Index | Thread Index | Old Index