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

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Date:   Fri, 4 Sep 2020 10:30:48 -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 9/4/20 9:59 AM, Jesper Dangaard Brouer wrote:
>> dev_rx for example seems like it could just be the netdev
>> index rather than a pointer or perhaps can be removed completely. I
>> believe it is only used for 1 use case (redirects to CPUMAP); maybe that
>> code can be refactored to handle the dev outside of xdp_frame.
> 
> The dev_rx is needed when creating an SKB from a xdp_frame (basically
> skb->dev = rx_dev). Yes, that is done in cpumap, but I want to
> generalize this.  The veth also creates SKBs from xdp_frame, but use
> itself as skb->dev.
> 
> And yes, we could save some space storing the index instead, and trade
> space for cycles in a lookup.

I think this can be managed without adding a reference to the xdp_frame.
I'll start a separate thread on that.

>>
>> As for frame_sz, why does it need to be larger than a u16?
> 
> Because PAGE_SIZE can be 64KiB on some archs.
> 

ok, is there any alignment requirement? can frame_sz be number of 32-bit
words? I believe bit shifts are cheap.