4 ms·

This is really cool. I wonder about whether or not putting the ISA itself directly in hardware is the right path forward, versus "virtualizing" it and compiling

by aseipp 5y ago

This is really cool. I wonder about whether or not putting the ISA itself directly in hardware is the right path forward, versus "virtualizing" it and compiling the bytecode again down to (yet another) architecture. It's a much better fit for hardware than a lot of "virtual" ISAs, though. There are some reasons you might do this, but then again, reasons not to. Netronome uses this virtual approach instead of baking eBPF into their processors, from my cursory understanding. An advantage of that is agility; FPGAs are only so agile compared to a software compiler and on top of that new evolving features have impacts on things like clock frequency, testing, etc. Beyond that there are also some big "what ifs" about eBPF; for example the fact it just uses the underlying Linux kernel memory model for concurrency. Maybe this is good. Maybe this is bad. I don't know. But it has a big impact on the underlying design of the memory subsystem for any real design. Intuitively I want to say the concurrency abstractions in eBPF should have always been much, much higher level from the start for reasons like this (for example, maybe it would have just been easier to flat-out introduce the concept of transactions, if you're already going to write a verifier to check every incoming program. This is much more difficult than what the verifier does today, but it's also a very well explored one and makes the machine memory model irrelevant. It's not the kind of stuff kernel developers do or think about, though...) But it's not, so it's still definitely something to think about. Do alternative vendors handle this already if they offload programs? There is also the current state we're in where "what is a valid eBPF program" mostly just comes down to "whatever the Linux kernel says is a valid eBPF program", so you can probably get into situations where code emitted by Clang will work in one of the two, and not the another (note: you can already get into situations where Clang will emit code from valid input that the kernel then rejects, so this probably isn't a very difficult example to construct.) This project already has some differences in ABI versus Linux, though, so... Overall this is still a really cool project, though. You'd have to do something like this to answer all the above, and it bills itself as an experiment. You could still have a lot of fun with this even ignoring all that. And it doesn't use Verilog. 10/10.

ilovecaching 5y ago
> for example, maybe it would have just been easier to flat-out introduce the concept of transactions, if you're already going to write a verifier to check every incoming program Could you elaborate on this? Loading the program is a one time operation. The actual evocation is dependent on the program type and its location in the kernel. I do not believe there is a blanket way to describe how eBPF interacts with the VM or its concurrency model. To your second point about verification, yes this is something that is actively being worked on. with BTF and CO-RE we've made huge strides over module loading, however, and eBPF already has many advantages over use cases where people were forced to load bespoke kernel bypass modules.
- aseipp 5y ago
  I mean that, instead of exposing atomic operations in the programming model (which are imperative constructs from the CPU to write to a memory location and require a notion of a memory model), the BPF virtual ISA could have just given you an ability to execute a series of high-level actions atomically or have them fail. It is basically a very small implementation of the ACI in ACID. This is only practical in this case because an eBPF program would need to be verified (by a trusted component; in this case the kernel verifier) before it can be executed anyway; any normal language with unverified, unrestricted semantics is too difficult to bolt transactions onto. You can just define a statically-determinable subset of programs that work and are accepted. This approach would have both upsides and downsides. The downsides include the need for more precise semantics, and an increase in verifier complexity (a lesser downside is an increase in compiler complexity to compile transactions to the underlying ISA; but I consider this 'lesser' because the verifier is more important, overall.) The upsides include a nicer programming model for users and independence from the underlying memory model in the processor, which gives you a lot of degrees of freedom to attack the problem. I don't know what you would do in the case of like, livelock though. But I also don't know what the failure mode is if, say, the program I ran today always ran and hit some dynamic error, either e.g. in the case of livelock you have to eventually just kill the program to make any further forward progress, but this seems no different than if an eBPF program simply always failed due to dynamic constraints. If the program just keeps exploding you eventually have to back off... Another downside I guess is that it's probably hard to implement transactional models in raw RTL, like this project :) But this is all just an idea. The thrust of it is to turn small exposures of the underlying processor semantics into higher-level compiler problems. I think this would work because you don't need to solve every compiler problem perfectly, just a small subset of them for a small set of domains. It doesn't really matter for Linux today because it just picked a common subset of the memory model for eBPF, but if anyone was to design a newer language for things like this in some new system, maybe some ideas like that are worthwhile. After all, CO-RE and related projects are about bringing "portability" to BPF programs. This is just an (somewhat logical) extension of that idea to the concurrent programming model. I don't need to worry about processor consistency models when I type "COMMIT TRANSACTION" in a SQL prompt, after all. > To your second point about verification, yes this is something that is actively being worked on. It's sort of hard to solve right now in the "Will this arbitrary C program work or not" sense, because the underlying toolchain and kernel implementation is evolving. I know there's a userspace verifier. What would be excellent is something like the ability to compile the kernel implementation of the verifier, but in userspace, so you could use it in CI. Does this already work, maybe?
KMag 5y ago
Speaking of alternative representations of eBPF programs, one thing I've noticed about eBPF is that it's particularly well suited to asymmetric number system (ANS)-based compression (such as Facebook's zstd). Asymmetric number systems require compression of buffers in the opposite order that they're decompressed. In eBPF, all branches need to jump forward, so if you're compressing the byte stream backwards, you always already know the offset in the compression stream of all valid branch targets. You'd need a first pass to find branch targets, so that you only remember the necessary bookkeeping information for branch targets as you're compressing, and the representation of branch targets takes some more space (an offset plus internal state of the ANS state machine), but a combination of eBPF and ANS-based compression still seems like a good match for a bytecode that's executed while it's being decompressed. The table-based variants of ANS seem particularly well suited to hardware/FPGA implementation. I remember some papers IBM published showing good results in improving POWER code density for embedded applications by building some hard-wired Huffman tree based decompression into the MMU.
- monocasa 5y ago
  > In eBPF, all branches need to jump forward, That was true of cBPF, but eBPF simply requires that the kernel's verifier could convert the control flow graph into a DAG. So notably certain types of bounded loops are allowed, and generally both back and forward branches are allowed.
  - KMag 5y ago
    Oh, interesting. But if the DAG requirement still stands, then it can still be converted into forward-only branches. Why not force that work into the bytecode compiler (or macro assembler) and simplify the kernel logic?
    - monocasa 5y ago
      Because bounded loops are functionally a compression scheme on the graph. It's a tradeoff for sure though. Personally (and going way off into the weeds, these are my thoughts and not at all representative of people working on BPF) I'd also like to see other schemes eventually that also don't necessarily require verifiable halting semantics for correctness. One example: something like io_uring, but instead there's a program running just barely in kernel space, accounted to the host user thread's scheduling, issuing new syscalls already on the kernel side of the syscall boundary.

by aseipp 5y ago

ilovecaching 5y ago

> for example, maybe it would have just been easier to flat-out introduce the concept of transactions, if you're already going to write a verifier to check every incoming program Could you elaborate on this? Loading the program is a one time operation. The actual evocation is dependent on the program type and its location in the kernel. I do not believe there is a blanket way to describe how eBPF interacts with the VM or its concurrency model. To your second point about verification, yes this is something that is actively being worked on. with BTF and CO-RE we've made huge strides over module loading, however, and eBPF already has many advantages over use cases where people were forced to load bespoke kernel bypass modules.

aseipp 5y ago
I mean that, instead of exposing atomic operations in the programming model (which are imperative constructs from the CPU to write to a memory location and require a notion of a memory model), the BPF virtual ISA could have just given you an ability to execute a series of high-level actions atomically or have them fail. It is basically a very small implementation of the ACI in ACID. This is only practical in this case because an eBPF program would need to be verified (by a trusted component; in this case the kernel verifier) before it can be executed anyway; any normal language with unverified, unrestricted semantics is too difficult to bolt transactions onto. You can just define a statically-determinable subset of programs that work and are accepted. This approach would have both upsides and downsides. The downsides include the need for more precise semantics, and an increase in verifier complexity (a lesser downside is an increase in compiler complexity to compile transactions to the underlying ISA; but I consider this 'lesser' because the verifier is more important, overall.) The upsides include a nicer programming model for users and independence from the underlying memory model in the processor, which gives you a lot of degrees of freedom to attack the problem. I don't know what you would do in the case of like, livelock though. But I also don't know what the failure mode is if, say, the program I ran today always ran and hit some dynamic error, either e.g. in the case of livelock you have to eventually just kill the program to make any further forward progress, but this seems no different than if an eBPF program simply always failed due to dynamic constraints. If the program just keeps exploding you eventually have to back off... Another downside I guess is that it's probably hard to implement transactional models in raw RTL, like this project :) But this is all just an idea. The thrust of it is to turn small exposures of the underlying processor semantics into higher-level compiler problems. I think this would work because you don't need to solve every compiler problem perfectly, just a small subset of them for a small set of domains. It doesn't really matter for Linux today because it just picked a common subset of the memory model for eBPF, but if anyone was to design a newer language for things like this in some new system, maybe some ideas like that are worthwhile. After all, CO-RE and related projects are about bringing "portability" to BPF programs. This is just an (somewhat logical) extension of that idea to the concurrent programming model. I don't need to worry about processor consistency models when I type "COMMIT TRANSACTION" in a SQL prompt, after all. > To your second point about verification, yes this is something that is actively being worked on. It's sort of hard to solve right now in the "Will this arbitrary C program work or not" sense, because the underlying toolchain and kernel implementation is evolving. I know there's a userspace verifier. What would be excellent is something like the ability to compile the kernel implementation of the verifier, but in userspace, so you could use it in CI. Does this already work, maybe?

KMag 5y ago

Speaking of alternative representations of eBPF programs, one thing I've noticed about eBPF is that it's particularly well suited to asymmetric number system (ANS)-based compression (such as Facebook's zstd). Asymmetric number systems require compression of buffers in the opposite order that they're decompressed. In eBPF, all branches need to jump forward, so if you're compressing the byte stream backwards, you always already know the offset in the compression stream of all valid branch targets. You'd need a first pass to find branch targets, so that you only remember the necessary bookkeeping information for branch targets as you're compressing, and the representation of branch targets takes some more space (an offset plus internal state of the ANS state machine), but a combination of eBPF and ANS-based compression still seems like a good match for a bytecode that's executed while it's being decompressed. The table-based variants of ANS seem particularly well suited to hardware/FPGA implementation. I remember some papers IBM published showing good results in improving POWER code density for embedded applications by building some hard-wired Huffman tree based decompression into the MMU.

monocasa 5y ago
> In eBPF, all branches need to jump forward, That was true of cBPF, but eBPF simply requires that the kernel's verifier could convert the control flow graph into a DAG. So notably certain types of bounded loops are allowed, and generally both back and forward branches are allowed.
- KMag 5y ago
  Oh, interesting. But if the DAG requirement still stands, then it can still be converted into forward-only branches. Why not force that work into the bytecode compiler (or macro assembler) and simplify the kernel logic?
  - monocasa 5y ago
    Because bounded loops are functionally a compression scheme on the graph. It's a tradeoff for sure though. Personally (and going way off into the weeds, these are my thoughts and not at all representative of people working on BPF) I'd also like to see other schemes eventually that also don't necessarily require verifiable halting semantics for correctness. One example: something like io_uring, but instead there's a program running just barely in kernel space, accounted to the host user thread's scheduling, issuing new syscalls already on the kernel side of the syscall boundary.