manyworlds

Ah okay, fine grained control of cache to minimize IO waiting is a good counterpoint.

I actually have a lot of experience in this area, and I can say that effective readahead is a bit of a crapshoot. Only really works in trivial cases. Ultimately if IO latency sucks, nothing can save you.

His particular point doesn’t fully make sense either. It’s easy to kick off readahead when you only have access to block data, the kernel won’t issue redundant IO requests for blocks already in the cache. Also mlock/madvise give a lot of control in terms of dictating eviction strategies for special blocks.

All thins equal (costless syscall/mm swapping, IO), I still think inter-task synchronization is the largest overhead, but I have no numbers to back it up. Something tells me Marshalling all IO syscalls to a kernel thread would be about as slow as a user-space FUSE task.

Thanks for the examples but those all still seem like things vast majority of Linux users can do today, since vast majority of Linux users have root access. Both desktop and server.

Mobile users like android don’t have root but I don’t see why an untrusted mobile app would need bpf.

Only benefit of allowing non-root that I can see is enabling untrusted containers in cloud environments to do the same. All large cloud providers use KVM/zen (not containers) for untrusted users in which case they already have root.

Can you give an example of a scenario where the user doesn’t have root yet still would want to do those things?

Sure but I don’t see any compelling use cases to motivate opening it up. Do you have an example of one?

Even if the kernel opens it up without a compelling use case, it seems likely that distribution policy will keep it default locked to root.

Which is my point here. I don’t see a compelling reason to have a JIT in the kernel when AOT BPF seems to cover 90% of all existing use cases. In fact I may even write a bpf to kernel module compiler myself.

Hmm I don’t think this is the case, at least not when comparing fuse to in-kernel file systems. Having access to native VM structures only helps to the extent that you can avoid copies, yet in fuse, only one extra copy takes place. I think having to switch tasks (and associated work: swapping mm, flushing tlb, ireting/syscalling, synchronization) is really what kills perf

Unikernel or rump kernel would be better for that purpose

Hmm I think bpf today is used in fully trusted environments. Kernel level verification is unnecessary except in untrusted containerized environments or when running untrusted applications, both use cases being relatively rare/specialized.

I think the main benefit of bpf is that it prevents you from shooting yourself in the foot. Running totally untrusted code in the kernel just seems like a recipe for disaster and for that reason it makes sense bpf is still limited to root.

A BPF->kernel module compiler would ensure all necessary cleanup happens in __exit automatically

Generally kernel code is also written to specific constraints. Out-of-tree drivers are usually bad at conforming and hypothetical out-of-tree webassembly drivers would be bad for similar reasons. Memory safety isn’t enough, using kernel interfaces safely requires conformance that webassembly can’t statically guarantee like BPF can

Basic examples: code that takes a lock and never releases it, code that loops forever, code that leaks kernel resources.

He’s not off the mark completely, you can maintain production safety by using a BPF->kernel module compiler. Unnecessary to have an entire JIT infrastructure in the kernel just to get the safety benefits of BPF.

This is less about BPF vs native code, and more about the process model vs the event based model of application programming.

Event based handling is inherently more efficient because it runs in the context of the caller, instead of requiring its own context like in process-based applications.

This is the main reason why file system code in the kernel is more efficient than file system servers running in a different process, eg via FUSE. No context switching

Doubtful. Webassembly is Turing complete, BPF isn’t. Running untrusted unbounded code in the kernel is not smart. BPF was invented with kernel constraints in mind, webassembly was invented with browser constraints in mind. Completely different use cases

You can maintain production safety by using a BPF->kernel module compiler.

This additionally removes the need to have the bpf compiler in the kernel, reducing both core size and vulnerability surface area.

No reason BPF must imply JIT