netdev - Re: [PATCH bpf-next 4/6] bpf: cpumap: add the possibility to attach an eBPF program to cpumap

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Date:   Tue, 2 Jun 2020 11:29:34 +0200
From:   Lorenzo Bianconi <lorenzo@...nel.org>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, davem@...emloft.net,
        ast@...nel.org, toke@...hat.com, daniel@...earbox.net,
        lorenzo.bianconi@...hat.com, dsahern@...nel.org
Subject: Re: [PATCH bpf-next 4/6] bpf: cpumap: add the possibility to attach
 an eBPF program to cpumap

> On Sun, 31 May 2020 23:46:49 +0200
> Lorenzo Bianconi <lorenzo@...nel.org> wrote:
> 
> > diff --git a/kernel/bpf/cpumap.c b/kernel/bpf/cpumap.c
> > index 57402276d8af..24ab0a6b9772 100644
> > --- a/kernel/bpf/cpumap.c
> > +++ b/kernel/bpf/cpumap.c
> > @@ -51,6 +51,10 @@ struct xdp_bulk_queue {
> >  /* CPUMAP value */
> >  struct bpf_cpumap_val {
> >  	u32 qsize;	/* queue size */
> > +	union {
> > +		int fd;	/* program file descriptor */
> > +		u32 id;	/* program id */
> > +	} prog;
> >  };
>   
> Please name the union 'bpf_prog' and not 'prog'.
> We should match what David Ahern did for devmap.

Hi Jesper,

ack, I will align the struct to David's one in v2.

Regards,
Lorenzo

> 
> Even-though we are NOT exposing this in the UAPI header-file, this still
> becomes a UAPI interface (actually kABI).  The struct member names are
> still important, even-though this is a binary layout, because the BTF
> info is basically documenting this API.
> 
> Notice when kernel is compiled with BTF info, you (or end-user) can use
> pahole to "reverse" the struct layout (comments don't survive, so we
> need descriptive member names):
> 
> $ pahole bpf_devmap_val
> struct bpf_devmap_val {
> 	__u32                      ifindex;              /*     0     4 */
> 	union {
> 		int                fd;                   /*     4     4 */
> 		__u32              id;                   /*     4     4 */
> 	} bpf_prog;                                      /*     4     4 */
> 	struct {
> 		unsigned char      data[24];             /*     8    24 */
> 	} storage;                                       /*     8    24 */
> 
> 	/* size: 32, cachelines: 1, members: 3 */
> 	/* last cacheline: 32 bytes */
> };
> 
> -- 
> Best regards,
>   Jesper Dangaard Brouer
>   MSc.CS, Principal Kernel Engineer at Red Hat
>   LinkedIn: http://www.linkedin.com/in/brouer
> 
> 
> $ bpftool btf dump file /sys/kernel/btf/vmlinux format c | grep -A10 'struct bpf_devmap_val {'
> struct bpf_devmap_val {
> 	__u32 ifindex;
> 	union {
> 		int fd;
> 		__u32 id;
> 	} bpf_prog;
> 	struct {
> 		unsigned char data[24];
> 	} storage;
> };
> 

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