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Date: Tue, 24 Nov 2009 21:54:58 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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Subject: [Qemu-devel] Re: [PATCH] qemu/virtio-net: remove wrong s/g layout
	assumptions
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To: Anthony Liguori <aliguori@linux.vnet.ibm.com>
Cc: Shirley Ma <mashirle@us.ibm.com>, Rusty Russell <rusty@rustcorp.com.au>, qemu-devel@nongnu.org

On Tue, Nov 24, 2009 at 01:50:25PM -0600, Anthony Liguori wrote:
> Michael S. Tsirkin wrote:
>> virtio net currently assumes that the first s/g element it gets is
>> always virtio net header. This is wrong.
>> There should be no assumption on sg boundaries.  For example, the guest
>> should be able to put the virtio_net_hdr in the front of the skbuf data
>> if there is room.  Get rid of this assumption, properly consume space
>> from iovec, always.
>>   
>
> Practically speaking, we ought to advertise a feature bit to let a  
> kernel know that we are no longer broken.
>
> Otherwise, there are a ton of old userspaces that will break with new  
> guests.

My thinking is, first of all let's fix the bug.
We'll add a feature bit when or if some guest wants to use it.
Maybe this will be 100 years down the road when all old userspace
has died a natural death :)
Makes sense?

> -- 
> Regards,
>
> Anthony Liguori