Re: [PATCH 1/1] Defer skb allocation for both mergeable buffers and big packets in virtio_net

Anthony Liguori
2009-11-24T14:36:34Z

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