services

Services

JAM Services consist of PVM Byte code placed on-chain with 4 entry points.
To support the testing of these JAM Services, JAM Implementation teams are encouraged to place JAM Service code, in any language here. Note that JAM Service code is not necessary for teams to collaborate on JAM testnet, but it may support useful debugging. Services may include a bootstrap service included in genesis state.

• Raw assembly, with polkatool
• Rust, see polkatool
• C/C++, see JAMBrains
• Solidity, see revive

Having a battery of services and workpackages testing a wide variety of simple JAM Services will be invaluable for testing JAM implementations of PVM and support experimentation with the JAM refine-accumulate workflow.

Each JAM Service should have raw source code, build instructions, and a JAM-ready polkavm code blob with 4 entry points:

• is_authorized (entry point 0)
• refine (entry point 5)
• accumulate (entry point 10)
• on_transfer (entry point 15)

Furthermore, a bootstrap service is needed in genesis state to support many different privileged operations:

• create JAM Services
• assigning coretime
• requesting preimages
• changing the validator keysets

These always-accumulate/privileged services use the same JAMSNP work package submission+sharing processes as non-privileged service. Teams are encouraged to share Bootstrap service code along with their genesis configs.

Raw PVM Assembly code

Teams can develop PVM byte code from assembly following this method to generate code by hand as described here:

$ git clone https://github.com/koute/polkavm.git
$ cd polkavm
$ cargo run -p polkatool -- assemble tools/spectool/spec/src/inst_branch_greater_or_equal_signed_ok.txt -o output.polkavm
$ cargo run -p polkatool disassemble --show-raw-bytes output.polkavm

This will output the program in a PolkaVM-specific container (which is not part of the GP), but you can extract the code blob with a simple Rust program - use polkavm_common::program::ProgramParts::from_bytes to load the blob and then the code_and_jump_table field will have the raw program bytes. This is explored further below.

Rust using polkatool

Following the advice here, the process is documented in PR #176 - add polkatool jam-service support:

1. Setup toolchain

Install this rustc-rv32e-toolchain -- we found the release build sufficient.

After installation you should have ~/.rustup/toolchains/rve-nightly/. Then

export RUSTUP_TOOLCHAIN=rve-nightly

will make this tool chain accessible.

2. Compile JAM Service

cargo build --release --target-dir=./target

This will generate a 1MB "elf" directory in your target directory matching the above tool, e.g

# ls -l target/riscv32ema-unknown-none-elf/release/fib
-rwxr-xr-x 2 root root 1067744 Sep 26 09:37 target/riscv32ema-unknown-none-elf/release/fib

3. Generate PVM Byte code with polkatool

You can then use polkatool to generate "JAM-ready" PVM byte code and raw code blobs with:

# cargo run -p polkatool jam-service fib/target/riscv32ema-unknown-none-elf/release/jam-service-fib -o fib/jam_service.pvm -d fib/blob.pvm
warning: /root/go/src/github.com/colorfulnotion/polkavm/Cargo.toml: unused manifest key: workspace.lints.rust.unexpected_cfgs.check-cfg
    Finished dev [unoptimized + debuginfo] target(s) in 0.08s
     Running `target/debug/polkatool jam-service guest-programs/jam-service-fib/target/riscv32ema-unknown-none-elf/release/jam-service-fib -o guest-programs/jam-service-fib/jam_service.pvm -d guest-programs/jam-service-fib/blob.pvm`
Writing JAM-ready code blob "guest-programs/jam-service-fib/jam_service.pvm"
Writing raw code "guest-programs/jam-service-fib/blob.pvm"

The precise mechanism to have a JAM-ready code blob (as defined in GP) is under construction but you can see this in progress here

4. Disassemble

Given the above blob.pvm, you can disassemble it with polkatool:

# cargo run -p polkatool disassemble fib/blob.pvm  --show-raw-bytes
// RO data = 0/0 bytes
// RW data = 0/4128 bytes
// Stack size = 4096 bytes

// Instructions = 418
// Code size = 1185 bytes

      :                          @0
     0: 05 ee 01 00 00           jump @52  // is_authorized
      :                          @1
     5: 05 ed 01 00 00           jump @53  // refine
      :                          @2
    10: 05 af 03 00 00           jump @78  // accumulate
      :                          @3
    15: 05 d2 03                 jump @79  // on_transfer
      :                          @4


      :                          @52 [export #0: 'is_authorized']
   494: 04 07                    a0 = 0x0
   496: 13 00                    ret

...

C/C++

See JAMBrains/polkavm-examples for detailed instructions on how to build a JAM Service in C/C++, including host function calls. This is a WIP.

Solidity

revive is being developed to compile PVM code for Polkadot Asset Hub and may have JAM on the roadmap soon.