Show HN: DOOM in the kernel, or fibers in eBPF

24 pointsposted a day ago
by ayles
(ayles.github.io)

8 Comments

markrwilliams

33 minutes ago

Skip the first half of the article to learn how the project overcame eBPF programs' limited runtime environment:

"On Linux 6.9 and newer (6.10 on arm64, where JIT support for the arena landed later), the window is backed by bpf_arena... On kernels without a usable arena, the same four gigabytes are assembled from 4-MiB pieces... [in] separate global-data maps."

These maps are of type BPF_MAP_TYPE_ARRAY.

See https://lwn.net/Articles/961941/ for a discussion of `bpf_arena`.

ayles

22 minutes ago

And compatibility part was most fun =)

Basically, it is better to use maps that can be directly used with load with immediate map index/fd, but we have at most 64 of those, sharing limit with user's data maps and even maps used for (iirc) 7.1 gotox. So for most programs we just use 32 maps or so. If we need more memory - only then array maps (I mean one that has more than 1 element and each element is 4MB) are used as upper part of address space, since they require helper call and are overall slower for our case. But fiber stacks can be allocated from this "slow space", because fiber mostly never reloads its map pointer and fiber stack is aligned so it is always within one such region. (and that is one part of two - memory; second one is about compute and verifier limits and we overcome them by using regions and dispatch-loop)

tptacek

3 hours ago

There's so much good content in here, it's a shame you sandblasted your writeup with AI. "To the verifier, it is a number with a biography" is where I veered off the road and into a culvert.

ayles

3 hours ago

Yes, there is a language barrier. And I was in a hurry because of ongoing life events, so it was kinda "today or in a month" for me. Couldn't keep it much longer for myself and wanted to share.

tptacek

2 hours ago

I totally understand where you're coming from. I'd recommend just publishing in your own words even if English isn't your best language; at this point, in 2026, it will read as charming and genuine, not slapdash.

A big problem with current model AI writing is that the topic sentence of every paragraph reads like a magazine headline, and it can be tricky to catch because a lot of magazine headlines are kind of good --- when they're headlines!

Number with a biography, yeesh. :)

This was neat work, though. Thanks for sharing!

ayles

2 hours ago

Noted for the future. I actually wrote a draft myself and mostly used ai to translate it. And there were some "late discoveries" that required me to patch article fast. For instance, for older memory tier (without arenas) I was using 2MB+8B regions with 8B overlap - this allowed unaligned reads to happen without checks, but later I got 4MB non-overlapping regions with llvm read-write alignments to work and that allowed me to remove additional check on stores, make memory fully writable from host side and made atomics correct. So this kind of patch needed to land in article between my "finishing tinkerings".

rvz

3 hours ago

Damn. That is a real shame. This whole writeup has some slop in it then.

phishin

an hour ago

I appreciate adherence to the all caps requirement.