netdev - Re: [PATCH bpf-next] libbpf: remove zc variable as it is not used

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Message-ID: <f88c052e-bc75-9e70-6e94-3dc581baf5f4@mellanox.com>
Date:   Mon, 19 Aug 2019 06:47:20 +0000
From:   Maxim Mikityanskiy <maximmi@...lanox.com>
To:     Magnus Karlsson <magnus.karlsson@...il.com>,
        Jonathan Lemon <jonathan.lemon@...il.com>
CC:     Yonghong Song <yhs@...com>,
        Magnus Karlsson <magnus.karlsson@...el.com>,
        Björn Töpel <bjorn.topel@...el.com>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Network Development <netdev@...r.kernel.org>,
        bpf <bpf@...r.kernel.org>
Subject: Re: [PATCH bpf-next] libbpf: remove zc variable as it is not used

On 2019-08-19 09:35, Magnus Karlsson wrote:
> On Sat, Aug 17, 2019 at 12:02 AM Jonathan Lemon
> <jonathan.lemon@...il.com> wrote:
>>
>>
>>
>> On 16 Aug 2019, at 8:37, Yonghong Song wrote:
>>
>>> On 8/16/19 3:26 AM, Magnus Karlsson wrote:
>>>> The zc is not used in the xsk part of libbpf, so let us remove it.
>>>> Not
>>>> good to have dead code lying around.
>>>>
>>>> Signed-off-by: Magnus Karlsson <magnus.karlsson@...el.com>
>>>> Reported-by: Yonghong Song <yhs@...com> > ---
>>>>    tools/lib/bpf/xsk.c | 3 ---
>>>>    1 file changed, 3 deletions(-)
>>>>
>>>> diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
>>>> index 680e630..9687da9 100644
>>>> --- a/tools/lib/bpf/xsk.c
>>>> +++ b/tools/lib/bpf/xsk.c
>>>> @@ -65,7 +65,6 @@ struct xsk_socket {
>>>>       int xsks_map_fd;
>>>>       __u32 queue_id;
>>>>       char ifname[IFNAMSIZ];
>>>> -    bool zc;
>>>>    };
>>>>
>>>>    struct xsk_nl_info {
>>>> @@ -608,8 +607,6 @@ int xsk_socket__create(struct xsk_socket
>>>> **xsk_ptr, const char *ifname,
>>>>               goto out_mmap_tx;
>>>>       }
>>>>
>>>> -    xsk->zc = opts.flags & XDP_OPTIONS_ZEROCOPY;
>>>
>>> Since opts.flags usage is removed. Do you think it makes sense to
>>> remove
>>>           optlen = sizeof(opts);
>>>           err = getsockopt(xsk->fd, SOL_XDP, XDP_OPTIONS, &opts,
>>> &optlen);
>>>           if (err) {
>>>                   err = -errno;
>>>                   goto out_mmap_tx;
>>>           }
>>> as well since nobody then uses opts?
>>
>> IIRC, this was added specifically in
>> 2761ed4b6e192820760d5ba913834b2ba05fd08c
>> so that userland code could know whether the socket was operating in
>> zero-copy
>> mode or not.
> 
> Thanks for reminding me Jonathan.
> 
> Roping in Maxim here since he wrote the patch. Was this something you
> planned on using but the functionality that needed it was removed? The
> patch set did go through a number of changes in the libbpf area, if I
> remember correctly.
> 
> There are two options: either we remove it, or we add an interface in
> xsk.h so that people can use it. I vote for the latter since I think
> it could be useful. The sample app could use it at least :-).

+1, let's expose it and make xdpsock read and print it. I added this 
flag to libbpf when I added the ability to query it from the kernel, but 
I didn't pay attention that struct xsk_socket was private to libbpf, and 
xsk->zc couldn't be accessed externally.

> /Magnus
> 
>> --
>> Jonathan