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From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>
Cc: Andrii Nakryiko <andriin@fb.com>, bpf <bpf@vger.kernel.org>,
	Networking <netdev@vger.kernel.org>,
	Alexei Starovoitov <ast@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Kernel Team <kernel-team@fb.com>,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Jonathan Lemon <jonathan.lemon@gmail.com>
Subject: Re: [PATCH v2 bpf-next 4/7] libbpf: add BPF ring buffer support
Date: Fri, 22 May 2020 11:56:54 -0700
Message-ID: <CAEf4BzbWj28btDvzDJCJMpRJ55hiydV9c+F_pEFvuA4bMeb+Bg@mail.gmail.com> (raw)
In-Reply-To: <20200522011552.ak4dkxhqwg6j2koy@ast-mbp.dhcp.thefacebook.com>

On Thu, May 21, 2020 at 6:15 PM Alexei Starovoitov
<alexei.starovoitov@gmail.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;
> > +}
> > +

  reply index

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-17 19:57 [PATCH v2 bpf-next 0/7] BPF ring buffer Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 1/7] bpf: implement BPF ring buffer and verifier support for it Andrii Nakryiko
2020-05-19 12:57   ` kbuild test robot
2020-05-19 23:53   ` kbuild test robot
2020-05-22  0:25   ` Paul E. McKenney
2020-05-22 18:46     ` Andrii Nakryiko
2020-05-25 16:01       ` Paul E. McKenney
2020-05-25 18:45         ` Andrii Nakryiko
2020-05-22  1:07   ` Alexei Starovoitov
2020-05-22 18:48     ` Andrii Nakryiko
2020-05-25 20:34     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 2/7] tools/memory-model: add BPF ringbuf MPSC litmus tests Andrii Nakryiko
2020-05-22  0:34   ` Paul E. McKenney
2020-05-22 18:51     ` Andrii Nakryiko
2020-05-25 23:33       ` Andrii Nakryiko
2020-05-26  3:05         ` Paul E. McKenney
2020-05-17 19:57 ` [PATCH v2 bpf-next 3/7] bpf: track reference type in verifier Andrii Nakryiko
2020-05-22  1:13   ` Alexei Starovoitov
2020-05-22 18:53     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 4/7] libbpf: add BPF ring buffer support Andrii Nakryiko
2020-05-22  1:15   ` Alexei Starovoitov
2020-05-22 18:56     ` Andrii Nakryiko [this message]
2020-05-17 19:57 ` [PATCH v2 bpf-next 5/7] selftests/bpf: add BPF ringbuf selftests Andrii Nakryiko
2020-05-22  1:20   ` Alexei Starovoitov
2020-05-22 18:58     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 6/7] bpf: add BPF ringbuf and perf buffer benchmarks Andrii Nakryiko
2020-05-22  1:21   ` Alexei Starovoitov
2020-05-22 19:07     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 7/7] docs/bpf: add BPF ring buffer design notes Andrii Nakryiko
2020-05-22  1:23   ` Alexei Starovoitov
2020-05-22 19:08     ` Andrii Nakryiko
2020-05-25  9:59   ` Alban Crequy
2020-05-25 19:12     ` Andrii Nakryiko

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