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Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback

To: Keir Fraser <keir@xxxxxxxxxxxxx>
Subject: Re: [Xen-devel] RFC: [0/2] Remove netloop by lazy copying in netback
From: David Edmondson <dme@xxxxxxx>
Date: Tue, 20 Mar 2007 11:50:12 +0000
Cc: Xen Development Mailing List <xen-devel@xxxxxxxxxxxxxxxxxxx>, Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
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On Tue, Mar 20, 2007 at 10:27:48AM +0000, Keir Fraser wrote:
> On 20/3/07 10:22, "Keir Fraser" <keir@xxxxxxxxxxxxx> wrote:
> > One other thought I had -- does the Linux network stack ever modify date
> > residing outside the main skbuff data area (i.e., data in fragments)? If it
> > ever does, wouldn't your delayed copy in netback potentially race such
> > updates? If the Linux network stack guarantees never to do such a thing,
> > that would be nice for us. :-)
> Scrub that question: we already map the grants read-only, so the network
> stack can't be trying to write to fragments.

The Solaris stack expects to be able to write to received packets and
I have a hack in our front and back end drivers to permit a writable
mapping of front->back buffers in preference to a copy (it's declared
using a feature flag, so a guest that doesn't do it still works).

I've resisted meddling in the fault handler, but I guess that I should
spend some time there in case it's a straightforward addition...


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