linux-kernel - Re: [PATCH 1/1] Defer skb allocation for both mergeable buffers and big packets in virtio

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Message-ID: <4B0BEF70.5070104@codemonkey.ws>
Date:	Tue, 24 Nov 2009 08:36:32 -0600
From:	Anthony Liguori <anthony@...emonkey.ws>
To:	"Michael S. Tsirkin" <mst@...hat.com>
CC:	Rusty Russell <rusty@...tcorp.com.au>,
	Shirley Ma <mashirle@...ibm.com>,
	Eric Dumazet <eric.dumazet@...il.com>,
	Avi Kivity <avi@...hat.com>, netdev@...r.kernel.org,
	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Hollis Blanchard <hollisb@...ibm.com>
Subject: Re: [PATCH 1/1] Defer skb allocation for both mergeable buffers and
 big packets in virtio_net

Michael S. Tsirkin wrote:
> On Tue, Nov 24, 2009 at 08:54:23AM +1030, Rusty Russell wrote:
>   
>> On Tue, 24 Nov 2009 02:37:01 am Shirley Ma wrote:
>>     
>>>>> +             skb = (struct sk_buff *)buf;
>>>>>           
>>>> This cast is unnecessary, but a comment would be nice:
>>>>         
>>> Without this cast there is a compile warning. 
>>>       
>> Hi Shirley,
>>
>>    Looks like buf is a void *, so no cast should be necessary.  But I could
>> be reading the patch wrong.
>>
>>     
>>>> However, I question whether making it 16 byte is the right thing: the
>>>> ethernet header is 14 bytes long, so don't we want 8 bytes of padding?
>>>>         
>>> Because in QEMU it requires 10 bytes header in a separately, so one page
>>> is used to share between virtio_net_hdr header which is 10 bytes head
>>> and rest of data. So I put 6 bytes offset here between two buffers. I
>>> didn't look at the reason why a seperate buf is used for virtio_net_hdr
>>> in QEMU.
>>>       
>> It's a qemu bug.  It insists the header be an element in the scatterlist by
>> itself.  Unfortunately we have to accommodate it.
>>     
>
> We do?  Let's just fix this?
>   

So does lguest.  It's been that way since the beginning.  Fixing this 
would result in breaking older guests.

We really need to introduce a feature bit if we want to change this.

Regards,

Anthony Liguori
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