netdev - Re: [PATCH v2 bpf-next 4/7] libbpf: add BPF ring buffer support

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Date:   Fri, 22 May 2020 11:56:54 -0700
From:   Andrii Nakryiko <andrii.nakryiko@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Andrii Nakryiko <andriin@...com>, bpf <bpf@...r.kernel.org>,
        Networking <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <kernel-team@...com>,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Jonathan Lemon <jonathan.lemon@...il.com>
Subject: Re: [PATCH v2 bpf-next 4/7] libbpf: add BPF ring buffer support

On Thu, May 21, 2020 at 6:15 PM Alexei Starovoitov
<alexei.starovoitov@...il.com> wrote:
>
> On Sun, May 17, 2020 at 12:57:24PM -0700, Andrii Nakryiko wrote:
> > +
> > +static inline int roundup_len(__u32 len)
> > +{
> > +     /* clear out top 2 bits */
> > +     len <<= 2;
> > +     len >>= 2;
>
> what this is for?
> Overflow prevention?
> but kernel checked the size already?

No, as comment above says, just clearing upper two bits (busy bit and
discard bit). Busy bit should be 0, but discard bit could be set for
discarded record. I could have written this equivalently as:

len = (len & ~(1 << BPF_RINGBUF_DISCARD_BIT));

But I think compiler won't optimize it to two bit shifts, assuming bit
#31 might be set.

>
> > +     /* add length prefix */
> > +     len += BPF_RINGBUF_HDR_SZ;
> > +     /* round up to 8 byte alignment */
> > +     return (len + 7) / 8 * 8;
> > +}
> > +