netdev - Re: [RFC bpf-next 2/6] net: xdp: RX meta data infrastructure

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Message-ID: <20180706234249.dchucouomzwilytx@ast-mbp.dhcp.thefacebook.com>
Date:   Fri, 6 Jul 2018 16:42:51 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Jakub Kicinski <jakub.kicinski@...ronome.com>
Cc:     Daniel Borkmann <daniel@...earbox.net>,
        Saeed Mahameed <saeedm@...lanox.com>,
        "saeedm@....mellanox.co.il" <saeedm@....mellanox.co.il>,
        "alexander.h.duyck@...el.com" <alexander.h.duyck@...el.com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Tariq Toukan <tariqt@...lanox.com>,
        "john.fastabend@...il.com" <john.fastabend@...il.com>,
        "brouer@...hat.com" <brouer@...hat.com>,
        "borkmann@...earbox.net" <borkmann@...earbox.net>,
        "peter.waskiewicz.jr@...el.com" <peter.waskiewicz.jr@...el.com>
Subject: Re: [RFC bpf-next 2/6] net: xdp: RX meta data infrastructure

On Fri, Jul 06, 2018 at 02:33:58PM -0700, Jakub Kicinski wrote:
> On Fri, 6 Jul 2018 09:30:42 -0700, Alexei Starovoitov wrote:
> > On Thu, Jul 05, 2018 at 10:18:23AM -0700, Jakub Kicinski wrote:
> > > 
> > > I'm also not 100% on board with the argument that "future" FW can
> > > reshuffle things whatever way it wants to.  Is the assumption that
> > > future ASICs/FW will be designed to always use the "blessed" BTF
> > > format?  Or will it be reconfigurable at runtime?  
> > 
> > let's table configuration of metadata aside for a second.
> > 
> > Describing metedata layout in BTF allows NICs to disclose everything
> > NIC has to users in a standard and generic way.
> > Whether firmware is reconfigurable on the fly or has to reflashed
> > and hw powercycled to have new md layout (and corresponding BTF description)
> > is a separate discussion.
> > Saeed's proposal introduces the concept of 'offset' inside 'struct xdp_md_info'
> > to reach 'hash' value in metadata.
> > Essentially it's a run-time way to access 'hash' instead of build-time.
> > So bpf program would need two loads to read csum or hash field instead of one.
> > With BTF the layout of metadata is known to the program at build-time.
> > 
> > To reiterate the proposal:
> > - driver+firmware keep layout of the metadata in BTF format (either in the driver
> >   or driver can read it from firmware)
> > - 'bpftool read-metadata-desc eth0 > md_desc.h' command will query the driver and
> >   generate normal C header file based on BTF in the given NIC
> > - user does #include "md_desc.h" and bpf program can access md->csum or md->hash
> >   with direct single load out of metadata area in front of the packet
> > - llvm compiles bpf program and records how program is doing this md->csum accesses
> >   in BTF format as well (the compiler will be keeping such records
> >   for __sk_buff and all other structs too, but that's separate discussion)
> > - during sys_bpf(prog_load) the kernel checks (via supplied BTF) that the way the program
> >   accesses metadata (and other structs) matches BTF from the driver,
> >   so no surprises if driver+firmware got updated, but program is not recompiled
> > - every NIC can have their own layout of metadata and its own meaning of the fields,
> >   but would be good to standardize at least a few common fields like hash
> 
> Can I expose HW descriptors this way, though, or is the proposal to
> copy this data into the packet buffer?

That crossed my mind too. We can use BTF to describe HW descriptors too,
but I don't think it would buy us anything. AF_XDP approach is better.

> > Once this is working we can do more cool things with BTF.
> > Like doing offset rewriting at program load time similar to what we plan
> > to do for tracing. Tracing programs will be doing 'task->pid' access
> > and the kernel will adjust offsetof(struct task_struct, pid) during program load
> > depending on BTF for the kernel.
> > The same trick we can do for networking metadata.
> > The program will contain load instruction md->hash that will get automatically
> > adjusted to proper offset depending on BTF of 'hash' field in the NIC.
> > For now I'm proposing _not_ to go that far with offset rewriting and start
> > with simple steps described above.
> 
> Why? :(  Could we please go with the rewrite/driver helpers instead of
> impacting fast paths of the drivers yet again?  This rewrite should be
> easier than task->pid, because we have the synthetic user space struct
> xdp_md definition.

I don't understand 'impact fast path yet again' concern.
If NIC has certain metadata today, just derscribe what it has in BTF
and supply the actual per-packet md to xdp program as-is.
No changes for fast path at all.
Future rewritting will be done by the bpf/xdp core with zero
changes to the driver. All driver has to do is to provide BTF.