Re: [PATCH bpf-next] bpf/xdp: optimize bpf_xdp_pointer to avoid reading sinfo

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

Lorenzo Bianconi <lorenzo@kernel.org> writes:

>> Currently we observed a significant performance degradation in
>> samples/bpf xdp1 and xdp2, due XDP multibuffer "xdp.frags" handling,
>> added in commit 772251742262 ("samples/bpf: fixup some tools to be able
>> to support xdp multibuffer").
>> 
>> This patch reduce the overhead by avoiding to read/load shared_info
>> (sinfo) memory area, when XDP packet don't have any frags. This improves
>> performance because sinfo is located in another cacheline.
>> 
>> Function bpf_xdp_pointer() is used by BPF helpers bpf_xdp_load_bytes()
>> and bpf_xdp_store_bytes(). As a help to reviewers, xdp_get_buff_len() can
>> potentially access sinfo.
>> 
>> Perf report show bpf_xdp_pointer() percentage utilization being reduced
>> from 4,19% to 3,37% (on CPU E5-1650 @3.60GHz).
>> 
>> The BPF kfunc bpf_dynptr_slice() also use bpf_xdp_pointer(). Thus, it
>> should also take effect for that.
>> 
>> Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
>> ---
>>  net/core/filter.c |   12 ++++++++----
>>  1 file changed, 8 insertions(+), 4 deletions(-)
>> 
>> diff --git a/net/core/filter.c b/net/core/filter.c
>> index 968139f4a1ac..a635f537d499 100644
>> --- a/net/core/filter.c
>> +++ b/net/core/filter.c
>> @@ -3948,20 +3948,24 @@ void bpf_xdp_copy_buf(struct xdp_buff *xdp, unsigned long off,
>>  
>>  void *bpf_xdp_pointer(struct xdp_buff *xdp, u32 offset, u32 len)
>>  {
>> -	struct skb_shared_info *sinfo = xdp_get_shared_info_from_buff(xdp);
>>  	u32 size = xdp->data_end - xdp->data;
>> +	struct skb_shared_info *sinfo;
>>  	void *addr = xdp->data;
>>  	int i;
>>  
>>  	if (unlikely(offset > 0xffff || len > 0xffff))
>>  		return ERR_PTR(-EFAULT);
>>  
>> -	if (offset + len > xdp_get_buff_len(xdp))
>> -		return ERR_PTR(-EINVAL);
>> +	if (likely((offset < size))) /* linear area */
>> +		goto out;
>
> Hi Jesper,
>
> please correct me if I am wrong but looking at the code, in this way
> bpf_xdp_pointer() will return NULL (and not ERR_PTR(-EINVAL)) if:
> - offset < size
> - offset + len > xdp_get_buff_len()
>
> doing so I would say bpf_xdp_copy_buf() will copy the full packet starting from
> offset leaving some part of the auxiliary buffer possible uninitialized.
> Do you think it is an issue?

Yeah, you're right, bpf_xdp_load_bytes() should fail if trying to read
beyond the frame, and in this case it won't for non-frags; that's a
change in behaviour we probably shouldn't be making...

-Toke