netdev - Re: [PATCH bpf-next 2/3] bpf: btf: add btf json print functionality

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Date:   Thu, 21 Jun 2018 17:25:23 -0700
From:   Jakub Kicinski <jakub.kicinski@...ronome.com>
To:     Martin KaFai Lau <kafai@...com>
Cc:     Okash Khawaja <osk@...com>, Daniel Borkmann <daniel@...earbox.net>,
        "Alexei Starovoitov" <ast@...nel.org>, Yonghong Song <yhs@...com>,
        Quentin Monnet <quentin.monnet@...ronome.com>,
        "David S. Miller" <davem@...emloft.net>, <netdev@...r.kernel.org>,
        <kernel-team@...com>, <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH bpf-next 2/3] bpf: btf: add btf json print functionality

On Thu, 21 Jun 2018 16:58:15 -0700, Martin KaFai Lau wrote:
> On Thu, Jun 21, 2018 at 04:07:19PM -0700, Jakub Kicinski wrote:
> > On Thu, 21 Jun 2018 15:51:17 -0700, Martin KaFai Lau wrote:  
> > > On Thu, Jun 21, 2018 at 02:59:35PM -0700, Jakub Kicinski wrote:  
> > > > On Wed, 20 Jun 2018 13:30:53 -0700, Okash Khawaja wrote:    
> > > > > $ sudo bpftool map dump -p id 14
> > > > > [{
> > > > >         "key": 0
> > > > >     },{
> > > > >         "value": {
> > > > >             "m": 1,
> > > > >             "n": 2,
> > > > >             "o": "c",
> > > > >             "p": [15,16,17,18,15,16,17,18
> > > > >             ],
> > > > >             "q": [[25,26,27,28,25,26,27,28
> > > > >                 ],[35,36,37,38,35,36,37,38
> > > > >                 ],[45,46,47,48,45,46,47,48
> > > > >                 ],[55,56,57,58,55,56,57,58
> > > > >                 ]
> > > > >             ],
> > > > >             "r": 1,
> > > > >             "s": 0x7ffff6f70568,
> > > > >             "t": {
> > > > >                 "x": 5,
> > > > >                 "y": 10
> > > > >             },
> > > > >             "u": 100,
> > > > >             "v": 20,
> > > > >             "w1": 0x7,
> > > > >             "w2": 0x3
> > > > >         }
> > > > >     }
> > > > > ]    
> > > > 
> > > > I don't think this format is okay, JSON output is an API you shouldn't
> > > > break.  You can change the non-JSON output whatever way you like, but
> > > > JSON must remain backwards compatible.
> > > > 
> > > > The dump today has object per entry, e.g.:
> > > > 
> > > > {
> > > >         "key":["0x00","0x00","0x00","0x00",
> > > >         ],
> > > >         "value": ["0x02","0x00","0x00","0x00","0x00","0x00","0x00","0x00"
> > > >         ]
> > > > }
> > > > 
> > > > This format must remain, you may only augment it with new fields.  E.g.:
> > > > 
> > > > {
> > > >         "key":["0x00","0x00","0x00","0x00",
> > > >         ],
> > > > 	"key_struct":{
> > > > 		"index":0
> > > > 	},
> > > >         "value": ["0x02","0x00","0x00","0x00","0x00","0x00","0x00","0x00"
> > > >         ],
> > > > 	"value_struct":{
> > > > 		"src_ip":2,
> > > > 		"dst_ip:0
> > > > 	}
> > > > }    
> > > I am not sure how useful to have both "key|value" and "(key|value)_struct"
> > > while most people would prefer "key_struct"/"value_struct" if it is
> > > available.  
> > 
> > Agreed, it's not that useful, especially with the string-hex debacle :(
> > It's just about the backwards compat.
> >   
> > > How about introducing a new option, like "-b", to print the
> > > map with BTF (if available) such that it won't break the existing
> > > one (-j or -p) while the "-b" output can keep using the "key"
> > > and "value".
> > > 
> > > The existing json can be kept as is.  
> > 
> > That was my knee jerk reaction too, but on reflection it doesn't sound
> > that great.  We expect people with new-enough bpftool to use btf, so it
> > should be available in the default output, without hiding it behind a
> > switch.  We could add a switch to hide the old output, but that doesn't
> > give us back the names...  What about Key and Value or k and v?  Or
> > key_fields and value_fields?  
> I thought the current default output is "plain" ;)
> Having said that, yes, the btf is currently printed in json.
> 
> Ideally, the default json output should do what most people want:
> print btf and btf only (if it is available).
> but I don't see a way out without new option if we need to
> be backward compat :(
> 
> Agree that showing the btf in the existing json output will be useful (e.g.
> to hint people that BTF is available).  If btf is showing in old json,
> also agree that the names should be the same with the new json.
> key_fields and value_fields may hint it has >1 fields though.
> May be "formatted_key" and "formatted_value"?

SGTM!  Or even maybe as a "formatted" object?:

{
         "key":["0x00","0x00","0x00","0x00",
         ],
         "value": ["0x02","0x00","0x00","0x00","0x00","0x00","0x00","0x00"
         ],
	"formatted":{
	 	"key":{
 			"index":0
	 	},
	 	"value":{
 			"src_ip":2,
 			"dst_ip:0
	 	}
	}
}  

> > > > The name XYZ_struct may not be the best, perhaps you can come up with a
> > > > better one?  
> > > > 
> > > > Does that make sense?  Am I missing what you're doing here?
> > > > 
> > > > One process note - please make sure you run checkpatch.pl --strict on
> > > > bpftool patches before posting.
> > > > 
> > > > Thanks for working on this!