netdev - Re: [RFC PATCH v8 00/16] Provide a zero-copy method on KVM virtio-net.

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Message-ID: <1280764918.22830.7.camel@localhost.localdomain>
Date:	Mon, 02 Aug 2010 09:01:58 -0700
From:	Shirley Ma <mashirle@...ibm.com>
To:	Avi Kivity <avi@...hat.com>
Cc:	xiaohui.xin@...el.com, netdev@...r.kernel.org, kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org, mst@...hat.com, mingo@...e.hu,
	davem@...emloft.net, herbert@...dor.hengli.com.au,
	jdike@...ux.intel.com
Subject: Re: [RFC PATCH v8 00/16] Provide a zero-copy method on KVM
 virtio-net.

Hello Avi,

On Sun, 2010-08-01 at 11:18 +0300, Avi Kivity wrote:
> I don't understand.  Under what conditions do you use
> get_user_pages() 
> instead of get_user_pages_fast()?  Why?

The code always calls get_user_pages_fast, however, the page will be
unpinned in skb_free if the same page is not used again for a new
buffer. The reason for unpin the page is we don't want to pin all of the
guest kernel memory(memory over commit). So get_user_pages_fast will
call slow path get_user_pages. 

Your previous comment is suggesting to keep the page pinned for
get_user_pages_fast fast path?

Thanks
Shirley

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