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

[Jakub Kicinski <jakub.kicinski@netronome.com>]

On Mon, 26 Aug 2019 18:25:10 +0200, Daniel Borkmann wrote:
> 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.

Ack, there is a difference between co-author of code, and co-author as
step by step guidance. I've been doing this for 6 years now, and nobody
ever complained :)

Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>

Is that enough or should I repost?