Subject: Re: [RFC 07/12] nfp: add skb mark support to the bpf offload

From: Daniel Borkmann <[email-masked]>
Date: Thu, 02 Jun 2016 00:30:07 +0200

On 06/02/2016 12:19 AM, Jakub Kicinski wrote:
> On Wed, 1 Jun 2016 14:56:26 -0700, Alexei Starovoitov wrote:
>> On Wed, Jun 01, 2016 at 05:50:09PM +0100, Jakub Kicinski wrote:
>>> Skb marking should be set in designated register, FW will
>>> prepend it to the packet for us.
>>>
>>> Signed-off-by: Jakub Kicinski <[email-masked]>
>>> Reviewed-by: Dinan Gunawardena <[email-masked]>
>>> Reviewed-by: Simon Horman <[email-masked]>
>>> ---
>>>   drivers/net/ethernet/netronome/nfp/nfp_bpf_jit.c    | 20 ++++++++++++++++++++
>>>   drivers/net/ethernet/netronome/nfp/nfp_net.h        |  2 +-
>>>   drivers/net/ethernet/netronome/nfp/nfp_net_common.c |  8 +++++++-
>>>   3 files changed, 28 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/drivers/net/ethernet/netronome/nfp/nfp_bpf_jit.c b/drivers/net/ethernet/netronome/nfp/nfp_bpf_jit.c
>>> index d7eecfceba5c..b31e673a6fe8 100644
>>> --- a/drivers/net/ethernet/netronome/nfp/nfp_bpf_jit.c
>>> +++ b/drivers/net/ethernet/netronome/nfp/nfp_bpf_jit.c
>>> @@ -46,6 +46,8 @@
>>>
>>>   #define REG_IMM0_N	30 /* Bank AB */
>>>   #define REG_QNUM	29 /* Bank AB */
>>> +#define REG_MARK	28 /* Bank A */
>>> +#define REG_MARK_STS	28 /* Bank B */
>>>
>>>   /* --- NFP prog --- */
>>>   /* Foreach "multiple" entries macros provide pos and next<n> pointers.
>>> @@ -416,6 +418,15 @@ static int construct_data_ld(struct nfp_prog *nfp_prog, u16 offset, u8 size)
>>>   	return construct_data_ind_ld(nfp_prog, offset, 0, false, size);
>>>   }
>>>
>>> +static int wrp_skb_mark(struct nfp_prog *nfp_prog, u16 src)
>>> +{
>>> +	__emit_alu(nfp_prog, REG_MARK, ALU_DST_A, REG_NONE, ALU_OP_NONE, src,
>>> +		   false, false);
>>> +	__emit_immed(nfp_prog, REG_MARK_STS, ALU_DST_B, 1, false);
>>> +
>>> +	return 0;
>>> +}
>>> +
>>>   static int
>>>   construct_br_imm(struct nfp_prog *nfp_prog, u32 imm, u16 dst, u8 br, u16 off,
>>>   		 enum alu_op alu_op, bool sw)
>>> @@ -538,6 +549,14 @@ static int imm_ld8(struct nfp_prog *nfp_prog, struct nfp_insn_meta *meta)
>>>   	return 0;
>>>   }
>>>
>>> +static int mem_stx4(struct nfp_prog *nfp_prog, struct nfp_insn_meta *meta)
>>> +{
>>> +	if (meta->insn.off == offsetof(struct sk_buff, mark))
>>> +		return wrp_skb_mark(nfp_prog, meta->insn.src_reg * 2);
>>
>> couldn't figure out from the diff or commit log...
>> what is the meaning of 'skb->mark' for nfp?
>> Looks like it's writing into magic register and fw will do something
>> with that register?
>> 'mark' is packet metadata. Could you explain how it's passing
>> this metadata? Is it on the wire as well or somehow in the wire
>> only between two nfps?
>> Looks like interesting feature.
>
> Oh, it's not a magic register, it just an "API" I have between the BPF
> and the datapath firmware.  Whatever is put in that register will be
> prepended to the packet (if the mark status register is set).

That is very useful indeed!

Btw, do you later on plan to also add something similar like TC_ACT_REDIRECT,
f.e. to push the packet same or different NIC port out again w/o leaving the
HW?