Subject: Re: CVS commit: src/sys/arch/xen/xen
To: YAMAMOTO Takashi <>
From: Manuel Bouyer <>
List: source-changes
Date: 10/03/2005 15:57:59
On Mon, Oct 03, 2005 at 06:43:17PM +0900, YAMAMOTO Takashi wrote:
> >   to keep track of mapped pages, and use reference counters.
> - are you sure that it isn't possible to map a page several times?
>   to me, linux netback doesn't seem to do anything special for it.

When I tried it behaved just like if the previous mapping was removed
when the new mapping was added (trap in the kernel when trying to access the
old address). Now it's possible I did make a mistake, but anyway avoiding
entering extra mapping when possible is probably better, performance-wise.
With a NetBSD guest, i've seen up to 4 packets in the same page in a row,
2 is common. Theorically it can be up to 16 (PAGE_SIZE / MBYTES).

> - as you defer ack until xennetback_tx_free and the mbuf can live ~forever,
>   you can capture all buffers.
>   (i think that linux netback has the same problem.)

I wondered about this. Can the kernel really grab a mbuf forever ? For
which reason ?
An easy fix would be to switch back to copy when we are low on buffers.

Manuel Bouyer <>
     NetBSD: 26 ans d'experience feront toujours la difference