netdev - Re: [PATCH v2 net-next 1/9] xdp: introduce mb in xdp_buff/xdp

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Message-ID: <51b88537-877a-c6bb-b4fb-0d629f37c0e6@gmail.com>
Date:   Mon, 7 Sep 2020 19:22:39 -0600
From:   David Ahern <dsahern@...il.com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Lorenzo Bianconi <lorenzo@...nel.org>, netdev@...r.kernel.org,
        bpf@...r.kernel.org, davem@...emloft.net,
        lorenzo.bianconi@...hat.com, echaudro@...hat.com,
        sameehj@...zon.com, kuba@...nel.org, john.fastabend@...il.com,
        daniel@...earbox.net, ast@...nel.org, shayagr@...zon.com,
        David Ahern <dsahern@...nel.org>,
        Ilias Apalodimas <ilias.apalodimas@...aro.org>
Subject: Re: [PATCH v2 net-next 1/9] xdp: introduce mb in xdp_buff/xdp_frame

On 9/7/20 12:02 PM, Jesper Dangaard Brouer wrote:
> 
>> ok, is there any alignment requirement? can frame_sz be number of 32-bit
>> words? I believe bit shifts are cheap.
> 
> No that is not possible, because some drivers and generic-XDP have a
> fully dynamic frame_sz.
> 

frame_sz represents allocated memory right? What is the real range that
needs to be supported for frame_sz? Surely there is some upper limit,
and I thought it was 64kB.

Allocated memory will not be on an odd number, so fair to assume at a
minimum it is a multiple of 2. correct? At a minimum we should be able
to shift frame_sz by 1 which now covers 64kB in a u16.