netdev - Re: [PATCH bpf-next v4 01/16] bpf: implement lookup-free direct value access for maps

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Date:   Sat, 6 Apr 2019 09:54:51 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Daniel Borkmann <daniel@...earbox.net>
Cc:     bpf@...r.kernel.org, netdev@...r.kernel.org, ast@...nel.org,
        joe@...d.net.nz, yhs@...com, andrii.nakryiko@...il.com,
        kafai@...com
Subject: Re: [PATCH bpf-next v4 01/16] bpf: implement lookup-free direct
 value access for maps

On Sat, Apr 06, 2019 at 12:58:23PM +0200, Daniel Borkmann wrote:
> On 04/06/2019 03:56 AM, Alexei Starovoitov wrote:
> > On Fri, Apr 05, 2019 at 10:59:27PM +0200, Daniel Borkmann wrote:
> >>  
> >> -/* when bpf_ldimm64->src_reg == BPF_PSEUDO_MAP_FD, bpf_ldimm64->imm == fd */
> >> +/* When BPF ldimm64's insn[0].src_reg != 0 then this can have
> >> + * two extensions:
> >> + *
> >> + * insn[0].src_reg:  BPF_PSEUDO_MAP_FD   BPF_PSEUDO_MAP_VALUE
> >> + * insn[0].imm:      map fd              map fd
> >> + * insn[1].imm:      0                   offset into value
> >> + * insn[0].off:      0                   lower 16 bit of map index
> >> + * insn[1].off:      0                   higher 16 bit of map index
> >> + * ldimm64 rewrite:  address of map      address of map[index]+offset
> >> + * verifier type:    CONST_PTR_TO_MAP    PTR_TO_MAP_VALUE
> > ...
> >> +	else if (insn->src_reg == BPF_PSEUDO_MAP_VALUE)
> >> +		snprintf(dd->scratch_buff, sizeof(dd->scratch_buff),
> >> +			 "map[id:%u][%u]+%u", insn->imm,
> >> +			 ((__u32)(__u16)insn[0].off) |
> >> +			 ((__u32)(__u16)insn[1].off) << 16,
> >> +			 (insn + 1)->imm);
> > 
> > Hopefully one last nit...
> > Do we really need to allow this odd split index support?
> > Later patches enforce array of 1 element and libbpf only uses that.
> > This index feature feels too quirky and not really useful at this moment.
> > Can we enforce that insn[0|1].off == 0 instead ?
> > Later we can extend it to mean index without breaking anything.
> 
> I originally didn't have it in v2 of the series, but I ended up
> implementing it after feedback from Andrii back then complaining
> that it's too specific and not generic enough. I agreed with him
> that the limitation of max_elems = 1 wasn't too nice, so I went
> to implement that full 32 bit index can be used thus that it has
> the potential of efficient map lookup replacement for array maps in
> general which is quite nice since within single insn it allows to
> select index and offset into value all as simple 64 bit imm load.

I missed this discussion.
It sort-of sounds nice from kernel side, but how one can use it
from bpf program written in C ?
If it's assembler only feature, I'd rather not do it.
statc int ar1[N];
and 
static struct S { ...} ar2[M];
will still be normal map of 1 element from llvm side.
Right now there is no support for variable length access
into static vars. When it's added in the future the ar[var] will be
some base offset into map of 1 element plus register addition.
So no opportunity to use 'index'.
bpf_map_lookup_elem(map, &key); from program side passes
a pointer and it's a function call. Even if key is constant
register spill/fill due to function call caused perf loss,
so extra 'index' optimization won't buy much.
We can introduce some special intrinsic/builtin to support
this 'index' from C, but it's not pretty.
So far I couldn't come up with C example that can use such 'index' feature.