netdev - Re: [PATCH bpf-next 2/7] lib: reciprocal_div: implement the improved algorithm on the paper mentioned

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Message-ID: <20180626135258.7e80bead@cakuba.netronome.com>
Date:   Tue, 26 Jun 2018 13:52:58 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Song Liu <liu.song.a23@...il.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Daniel Borkmann <daniel@...earbox.net>,
        oss-drivers@...ronome.com, Networking <netdev@...r.kernel.org>,
        Jiong Wang <jiong.wang@...ronome.com>
Subject: Re: [PATCH bpf-next 2/7] lib: reciprocal_div: implement the
 improved algorithm on the paper mentioned

On Mon, 25 Jun 2018 23:21:10 -0700, Song Liu wrote:
> > +struct reciprocal_value_adv reciprocal_value_adv(u32 d, u8 prec)
> > +{
> > +       struct reciprocal_value_adv R;
> > +       u32 l, post_shift;
> > +       u64 mhigh, mlow;
> > +
> > +       l = fls(d - 1);
> > +       post_shift = l;
> > +       /* NOTE: mlow/mhigh could overflow u64 when l == 32 which means d has
> > +        * MSB set. This case needs to be handled before calling
> > +        * "reciprocal_value_adv", please see the comment at
> > +        * include/linux/reciprocal_div.h.
> > +        */  
> 
> Shall we handle l == 32 case better? I guess the concern here is extra
> handling may slow down the fast path? If that's the case, we should
> at least add a WARNING on the slow path.

Agreed, I think Jiong is travelling, hence no response.  We'll respin.