netdev - Re: [PATCH bpf-next 06/13] bpf: Introduce BPF_MAP_TYPE_STRUCT

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Message-ID: <20191220165158.bc6mp7w5ooof262h@kafai-mbp>
Date:   Fri, 20 Dec 2019 16:52:03 +0000
From:   Martin Lau <kafai@...com>
To:     Yonghong Song <yhs@...com>
CC:     "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        David Miller <davem@...emloft.net>,
        Kernel Team <Kernel-team@...com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>
Subject: Re: [PATCH bpf-next 06/13] bpf: Introduce  BPF_MAP_TYPE_STRUCT_OPS

On Thu, Dec 19, 2019 at 11:22:17PM -0800, Martin Lau wrote:

> [ ... ]
> 
> > > +/* __bpf_##_name (e.g. __bpf_tcp_congestion_ops) is the map's value
> > > + * exposed to the userspace and its btf-type-id is stored
> > > + * at the map->btf_vmlinux_value_type_id.
> > > + *
> > > + * The *_name##_dummy is to ensure the BTF type is emitted.
> > > + */
> > > +
> > >   #define BPF_STRUCT_OPS_TYPE(_name)				\
> > > -extern struct bpf_struct_ops bpf_##_name;
> > > +extern struct bpf_struct_ops bpf_##_name;			\
> > > +								\
> > > +static struct __bpf_##_name {					\
> > > +	BPF_STRUCT_OPS_COMMON_VALUE;				\
> > > +	struct _name data ____cacheline_aligned_in_smp;		\
> > > +} *_name##_dummy;
> > 
> > There are other ways to retain types in debug info without
> > creating new variables. For example, you can use it in a cast
> > like
> >      (void *)(struct __bpf_##_name *)v
> hmm... What is v?
Got it.  "v" could be any dummy pointer in a function.
I will use (void) instead of (void *) to avoid compiler warning.

> 
> > Not sure whether we could easily find a place for such casting or not.
This can be done in bpf_struct_ops_init().

Thanks for the tips!