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

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Date:   Sun, 25 Aug 2019 22:36:50 -0700
From:   Song Liu <liu.song.a23@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        bpf <bpf@...r.kernel.org>, Networking <netdev@...r.kernel.org>,
        oss-drivers@...ronome.com, Jiong Wang <jiong.wang@...ronome.com>
Subject: Re: [PATCH bpf] nfp: bpf: fix latency bug when updating stack index register

On Fri, Aug 23, 2019 at 7:04 PM Jakub Kicinski
<jakub.kicinski@...ronome.com> wrote:
>
> From: Jiong Wang <jiong.wang@...ronome.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@...ronome.com>
> Reviewed-by: Jakub Kicinski <jakub.kicinski@...ronome.com>
I haven't looked into the code yet. But ^^^ should be

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

right?