On 4/24/24 17:49, Alexei Starovoitov wrote:
On Wed, Apr 24, 2024 at 3:32 PM Kui-Feng Lee <sinquersw@xxxxxxxxx> wrote:
struct map_value {
struct {
struct task __kptr *p1;
struct thread __kptr *p2;
} arr[10];
};
won't be able to be represented as BPF_REPEAT_FIELDS?
BPF_REPEAT_FIELDS can handle it. With this case, bpf_parse_fields() will
create a list of btf_fields like this:
[ btf_field(type=BPF_KPTR_..., offset=0, ...),
btf_field(type=BPF_KPTR_..., offset=8, ...),
btf_field(type=BPF_REPEAT_FIELDS, offset=16, repeated_fields=2,
nelems=9, size=16)]
You might miss the explanation in [1].
btf_record_find() is still doing binary search. Looking for p2 in
obj->arr[1], the offset will be 24. btf_record_find() will find the
BPF_REPEATED_FIELDS one, and redirect the offset to
(field->offset - field->size + (16 - field->offset) % field->size) == 8
Then, it will return the btf_field whose offset is 8.
[1]
https://lore.kernel.org/all/4d3dc24f-fb50-4674-8eec-4c38e4d4b2c1@xxxxxxxxx/
I somehow completely missed that email.
Just read it and tbh it looks very unnatural and convoluted.
[kptr_a, kptr_b, repeated_fields(nelems=3, repeated_cnt=2),
repeated_fields(nelems=9, repeated_cnt=3)]
is kinda an inverted array description where elements come first
and then array type. I have a hard time imagining how search
in such thing will work.
About searching, it will find the elements if index is 0. For index >=
1, it will find repeated_fields(), and redirect to the offset to an
offset at index 0. The pseudo code looks like
field = bsearch(all_fields, offset..);
while (field && is_repeated_fields(field)) {
offset = redirect_offset(offset, field);
field =
bsearch(&all_fields[field.index-field.repeated_cnt..field.index], offset);
}
Also consider that arrays won't be huge, since bpf prog
can only access them with a constant offset.
Even array[NR_CPUS] is unlikely, since indexing into it
with a variable index won't be possible.
I also got a similar opinion from Andrii in another message.
So, I will move to flatten solution.
Thank you for your feedback.
