netdev - Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff if size is not enough

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Date:   Thu, 26 Oct 2017 09:28:00 -0600
From:   David Ahern <dsahern@...il.com>
To:     Stephen Hemminger <stephen@...workplumber.org>
Cc:     Hangbin Liu <liuhangbin@...il.com>, netdev@...r.kernel.org,
        Michal Kubecek <mkubecek@...e.cz>, Phil Sutter <phil@....cc>
Subject: Re: [PATCHv5 iproute2 net-next 2/2] lib/libnetlink: re malloc buff if
 size is not enough

On 10/26/17 4:24 AM, Stephen Hemminger wrote:
>>
>> The kernel needs a flag that says "give me the message of the buffer is
>> large enough; if not just PEEK and tell me the length." That would avoid
>> the double call in most cases.
> 
> Actually this has little impact because old code was doing implicit zero
> of whole buffer, new code does not.
> 

The patch calls recvmsg twice; libnl does the same thing. It would be
better performance wise to have a flag that allows retrieval of the
message if the supplied buffer is large enough and PEEK semantics if
not. It was really a comment on how we could do better with proper
kernel support.