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Re: Re: Re: Re: [Xen-devel] SetPageForeign in netback

  • To: "Zang Hongyong" <zanghongyong@xxxxxxxxxx>
  • From: "Grzegorz Miłoś" <gm281@xxxxxxxxx>
  • Date: Tue, 24 Jun 2008 07:41:49 +0100
  • Cc: xen-devel <xen-devel@xxxxxxxxxxxxxxxxxxx>
  • Delivery-date: Mon, 23 Jun 2008 23:42:13 -0700
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  • List-id: Xen developer discussion <xen-devel.lists.xensource.com>

>>The reason why you have to copy is to make sure that you pull the skb
>>into a fresh page (which doesn't have any other pieces of data in it).
>>It's only then that you can flip it to DomU.
>>It used to be the case that the backend was more efficient in the
>>rx-flip mode. Some/most skbs copies were avoided by keeping a close
>>eye on the skb allocation. Since rx-flip is now only/mostly kept for
>>backward compatibility (rx-copy is the default) the code was
> You mean in previous Xen release, rx-flip is more efficient than today's 
> release. And keeping rx-flip in Xen Source Code tree today is only for 
> compatibility. So no need to do more optimizing for it.
> Is my understanding right?

I don't know if much performance was lost (the code got simplified, at
the expense of potentially more skb copies, but I don't know of any
experimental tests to benchmark what the effect was exactly), but
aside from that, your understanding is correct.


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