RE: [RFC PATCH v7 01/19] Add a new structure for skb buffer from external.

Xin, Xiaohui
2010-06-18T07:14:18Z

>-----Original Message-----
>From: Herbert Xu [mailto:herbert@gondor.apana.org.au]
>Sent: Friday, June 18, 2010 1:59 PM
>To: Xin, Xiaohui
>Cc: Stephen Hemminger; netdev@vger.kernel.org; kvm@vger.kernel.org;
>linux-kernel@vger.kernel.org; mst@redhat.com; mingo@elte.hu; davem@davemloft.net;
>jdike@linux.intel.com; Rusty Russell
>Subject: Re: [RFC PATCH v7 01/19] Add a new structure for skb buffer from external.
>
>On Fri, Jun 18, 2010 at 01:26:49PM +0800, Xin, Xiaohui wrote:
>>
>> Herbert,
>> I have questions about the idea above:
>> 1) Since netdev_alloc_skb() is still there, and we only modify alloc_page(),
>> then we don't need napi_gro_frags() any more, the driver's original receiving
>> function is ok. Right?
>
>Well I was actually thinking about converting all drivers that
>need this to napi_gro_frags. But now that you mention it, yes
>we could still keep the old interface to minimise the work.
>
>> 2) Is napi_gro_frags() only suitable for TCP protocol packet?
>> I have done a small test for ixgbe driver to let it only allocate paged buffers
>> and found kernel hangs when napi_gro_frags() receives an ARP packet.
>
>It should work with any packet. In fact, I'm pretty sure the
>other drivers (e.g., cxgb3) use that interface for all packets.
>
Thanks for the verification. By the way, does that mean that nearly all drivers can use the
same napi_gro_frags() to receive buffers though currently each driver has it's own receiving
function?

>> 3) As I have mentioned above, with this idea, netdev_alloc_skb() will allocate
>> as usual, the data pointed by skb->data will be copied into the first guest buffer.
>> That means we should reserve sufficient room in guest buffer. For PS mode
>> supported driver (for example ixgbe), the room will be more than 128. After 128bytes,
>> we will put the first frag data. Look into virtio-net.c the function page_to_skb()
>> and receive_mergeable(), that means we should modify guest virtio-net driver to
>> compute the offset as the parameter for skb_set_frag().
>>
>> How do you think about this? Attached is a patch to how to modify the guest driver.
>> I reserve 512 bytes as an example, and transfer the header len of the skb in hdr->hdr_len.
>
>Expanding the buffer size to 512 bytes to accomodate PS mode
>looks reasonable to me.
>
>However, I don't think we should increase the copy threshold to
>512 bytes at the same time. I don't have any figures myself but
>I think if we are to make such a change it should be a separate
>one and come with supporting numbers.
>
Let me have a look to see if I can retain the copy threshold as 128 bytes
and copy the header data safely.

>Cheers,
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