netdev - Re: [RFC bpf-next 0/3] tools: bpftool: add subcommand to count map entries

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Date:   Wed, 14 Aug 2019 18:14:18 +0100
From:   Edward Cree <ecree@...arflare.com>
To:     Quentin Monnet <quentin.monnet@...ronome.com>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>, <bpf@...r.kernel.org>,
        <netdev@...r.kernel.org>, <oss-drivers@...ronome.com>
Subject: Re: [RFC bpf-next 0/3] tools: bpftool: add subcommand to count map
 entries

On 14/08/2019 17:58, Quentin Monnet wrote:
> 2019-08-14 17:45 UTC+0100 ~ Edward Cree <ecree@...arflare.com>
>> This might be a really dumb suggestion, but: you're wanting to collect a
>>  summary statistic over an in-kernel data structure in a single syscall,
>>  because making a series of syscalls to examine every entry is slow and
>>  racy.  Isn't that exactly a job for an in-kernel virtual machine, and
>>  could you not supply an eBPF program which the kernel runs on each entry
>>  in the map, thus supporting people who want to calculate something else
>>  (mean, min and max, whatever) instead of count?
>>
> Hi Edward, I like the approach, thanks for the suggestion.
>
> But I did not mention that we were using offloaded maps: Tracing the
> kernel would probably work for programs running on the host, but this is
> not a solution we could extend to hardware offload.
I don't see where "tracing" comes into it; this is a new program type and
 a new map op under the bpf() syscall.
Could the user-supplied BPF program not then be passed down to the device
 for it to run against its offloaded maps?