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Re: xvif(4) and kernel.org domU
Manuel Bouyer wrote:
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.
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
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.
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