Re: [PATCH bpf] nfp: bpf: fix latency bug when updating stack index register

[Daniel Borkmann <daniel@iogearbox.net>]

On 8/26/19 6:18 PM, Alexei Starovoitov wrote:
> On Mon, Aug 26, 2019 at 8:57 AM Jakub Kicinski
> <jakub.kicinski@netronome.com> wrote:
>> On Sun, Aug 25, 2019 at 10:37 PM Song Liu <liu.song.a23@gmail.com> wrote:
>>> On Fri, Aug 23, 2019 at 7:04 PM Jakub Kicinski wrote:
>>>> From: Jiong Wang <jiong.wang@netronome.com>
>>>>
>>>> NFP is using Local Memory to model stack. LM_addr could be used as base of
>>>> a 16 32-bit word region of Local Memory. Then, if the stack offset is
>>>> beyond the current region, the local index needs to be updated. The update
>>>> needs at least three cycles to take effect, therefore the sequence normally
>>>> looks like:
>>>>
>>>>    local_csr_wr[ActLMAddr3, gprB_5]
>>>>    nop
>>>>    nop
>>>>    nop
>>>>
>>>> If the local index switch happens on a narrow loads, then the instruction
>>>> preparing value to zero high 32-bit of the destination register could be
>>>> counted as one cycle, the sequence then could be something like:
>>>>
>>>>    local_csr_wr[ActLMAddr3, gprB_5]
>>>>    nop
>>>>    nop
>>>>    immed[gprB_5, 0]
>>>>
>>>> However, we have zero extension optimization that zeroing high 32-bit could
>>>> be eliminated, therefore above IMMED insn won't be available for which case
>>>> the first sequence needs to be generated.
>>>>
>>>> Fixes: 0b4de1ff19bf ("nfp: bpf: eliminate zero extension code-gen")
>>>> Signed-off-by: Jiong Wang <jiong.wang@netronome.com>
>>>> Reviewed-by: Jakub Kicinski <jakub.kicinski@netronome.com>
>>> I haven't looked into the code yet. But ^^^ should be
>>>
>>> Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
>>>
>>> right?
>>
>> I prefer Review on code I review, ack on code I ack, and sign-off on
>> code I co-author.
> 
> I believe if you're sending somebody else patch you have to add your SOB
> in addition to their 'Author:' and their SOB fields.

+1, for co-authoring there's a 'Co-authored-by:' tag which seems to be frequently
used these days.