linux-kernel - Re: [PATCH net-next v2 1/4] indirect call wrappers: helpers to speed-up indirect calls of builtin

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Date:   Fri, 07 Dec 2018 09:46:35 +0000
From:   David Woodhouse <dwmw2@...radead.org>
To:     Paolo Abeni <pabeni@...hat.com>, netdev@...r.kernel.org
Cc:     "David S. Miller" <davem@...emloft.net>,
        Eric Dumazet <eric.dumazet@...il.com>,
        Paul Turner <pjt@...gle.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next v2 1/4] indirect call wrappers: helpers to
 speed-up indirect calls of builtin

On Wed, 2018-12-05 at 19:13 +0100, Paolo Abeni wrote:
> +/*
> + * We can use INDIRECT_CALL_$NR for ipv6 related functions only if ipv6 is
> + * builtin, this macro simplify dealing with indirect calls with only ipv4/ipv6
> + * alternatives
> + */
> +#if IS_BUILTIN(CONFIG_IPV6)
> +#define INDIRECT_CALL_INET(f, f2, f1, ...) \
> +       INDIRECT_CALL_2(f, f2, f1, __VA_ARGS__)
> +#elif IS_ENABLED(CONFIG_INET)
> +#define INDIRECT_CALL_INET(f, f2, f1, ...) INDIRECT_CALL_1(f, f1, __VA_ARGS__)
> +#else
> +#define INDIRECT_CALL_INET(f, f2, f1, ...) f(__VA_ARGS__)
> +#endif
> +
> +#endif

Thanks for working on this.

I'm not stunningly keen on the part cited above. And it doesn't seem to
be working either, given Dave's later error and reversion.

I wonder if we can declare the common case functions as 'weak' so that
the link failures don't happen when they're absent.

Once we extend this past the network code, especially to file systems'
f_ops, I suspect we're going to want to use something like static keys
to patch the common cases at runtime — perhaps changing the f_ops
default according to what the root file system is, etc.

I'd quite like to see the API for this taking that into account even if
it's left to be a future development.

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