netdev - Re: [PATCH v3 net-next RFC] Generic XDP

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Message-Id: <20170417.154955.1624611510140672627.davem@davemloft.net>
Date:   Mon, 17 Apr 2017 15:49:55 -0400 (EDT)
From:   David Miller <davem@...emloft.net>
To:     brouer@...hat.com
Cc:     alexei.starovoitov@...il.com, kubakici@...pl,
        netdev@...r.kernel.org, xdp-newbies@...r.kernel.org
Subject: Re: [PATCH v3 net-next RFC] Generic XDP

From: Jesper Dangaard Brouer <brouer@...hat.com>
Date: Sun, 16 Apr 2017 22:26:01 +0200

> The bpf tail-call use-case is a very good example of why the
> verifier cannot deduct the needed HEADROOM upfront.

This brings up a very interesting question for me.

I notice that tail calls are implemented by JITs largely by skipping
over the prologue of that destination program.

However, many JITs preload cached SKB values into fixed registers in
the prologue.  But they only do this if the program being JITed needs
those values.

So how can it work properly if a program that does not need the SKB
values tail calls into one that does?

Daniel, Alexei?