Re: [PATCH net-next v4 12/19] xdp: add generic xdp_build_skb_from_buff()

["Toke Høiland-Jørgensen" <toke@redhat.com>]

Alexander Lobakin <aleksander.lobakin@intel.com> writes:

> From: Toke Høiland-Jørgensen <toke@redhat.com>
> Date: Mon, 11 Nov 2024 17:39:51 +0100
>
>> Alexander Lobakin <aleksander.lobakin@intel.com> writes:
>> 
>>> The code which builds an skb from an &xdp_buff keeps multiplying itself
>>> around the drivers with almost no changes. Let's try to stop that by
>>> adding a generic function.
>>> Unlike __xdp_build_skb_from_frame(), always allocate an skbuff head
>>> using napi_build_skb() and make use of the available xdp_rxq pointer to
>>> assign the Rx queue index. In case of PP-backed buffer, mark the skb to
>>> be recycled, as every PP user's been switched to recycle skbs.
>>>
>>> Signed-off-by: Alexander Lobakin <aleksander.lobakin@intel.com>
>>> ---
>>>  include/net/xdp.h |  1 +
>>>  net/core/xdp.c    | 55 +++++++++++++++++++++++++++++++++++++++++++++++
>>>  2 files changed, 56 insertions(+)
>>>
>>> diff --git a/include/net/xdp.h b/include/net/xdp.h
>>> index 4c19042adf80..b0a25b7060ff 100644
>>> --- a/include/net/xdp.h
>>> +++ b/include/net/xdp.h
>>> @@ -330,6 +330,7 @@ xdp_update_skb_shared_info(struct sk_buff *skb, u8 nr_frags,
>>>  void xdp_warn(const char *msg, const char *func, const int line);
>>>  #define XDP_WARN(msg) xdp_warn(msg, __func__, __LINE__)
>>>  
>>> +struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp);
>>>  struct xdp_frame *xdp_convert_zc_to_xdp_frame(struct xdp_buff *xdp);
>>>  struct sk_buff *__xdp_build_skb_from_frame(struct xdp_frame *xdpf,
>>>  					   struct sk_buff *skb,
>>> diff --git a/net/core/xdp.c b/net/core/xdp.c
>>> index b1b426a9b146..3a9a3c14b080 100644
>>> --- a/net/core/xdp.c
>>> +++ b/net/core/xdp.c
>>> @@ -624,6 +624,61 @@ int xdp_alloc_skb_bulk(void **skbs, int n_skb, gfp_t gfp)
>>>  }
>>>  EXPORT_SYMBOL_GPL(xdp_alloc_skb_bulk);
>>>  
>>> +/**
>>> + * xdp_build_skb_from_buff - create an skb from an &xdp_buff
>>> + * @xdp: &xdp_buff to convert to an skb
>>> + *
>>> + * Perform common operations to create a new skb to pass up the stack from
>>> + * an &xdp_buff: allocate an skb head from the NAPI percpu cache, initialize
>>> + * skb data pointers and offsets, set the recycle bit if the buff is PP-backed,
>>> + * Rx queue index, protocol and update frags info.
>>> + *
>>> + * Return: new &sk_buff on success, %NULL on error.
>>> + */
>>> +struct sk_buff *xdp_build_skb_from_buff(const struct xdp_buff *xdp)
>>> +{
>>> +	const struct xdp_rxq_info *rxq = xdp->rxq;
>>> +	const struct skb_shared_info *sinfo;
>>> +	struct sk_buff *skb;
>>> +	u32 nr_frags = 0;
>>> +	int metalen;
>>> +
>>> +	if (unlikely(xdp_buff_has_frags(xdp))) {
>>> +		sinfo = xdp_get_shared_info_from_buff(xdp);
>>> +		nr_frags = sinfo->nr_frags;
>>> +	}
>> 
>> Why this separate branch at the start of the function? nr_frags is no
>> used until the other branch below, so why not just make that branch on
>> xdp_buff_has_frags() and keep everything frags-related together in one
>> block?
>
> Because napi_build_skb() will call build_skb_around() which will
> memset() a piece of shared info including nr_frags.
> xdp_build_skb_from_frame() has the same logic. I'd be happy to have only
> one block, but I can't =\

Ah, right. Annoying, but OK :)

-Toke