Running Geth with an External VM¶

Geth supports the EVMC VM connector API Version 7 as an experimental feature. This interface provides support for external EVM and EWASM interpreters.

External interpreters can be configured on the command line via a --vm.-prefixed flag for normal instantiation, and --evmc. for testing.

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  --vm.evm value                      External EVM configuration (default = built-in interpreter)
  --vm.ewasm value                    External ewasm configuration (default = built-in interpreter)

Provided to these flags should be EWASM and EVM shared object libraries, as follows:

• --vm.ewasm=<path/to/interpreter.so
• --vm.evm=<path/to/interpreter.so

Testing EVMC Support¶

This implementation may be tested by following the command defined in the Makefile as test-evmc, which tests the implementation against the EWASM and EVM libraries mentioned above, against the /tests/ StateTest suite.

These tests run exclusively via Github Actions, configured at .github/workflows/evmc.yml.

Discussion: Customizing EVMC Configuration¶

While core-geth supports highly granular EIP/ECIP/xIP chain feature configuration (ie fork feature configs), EVMC does not. EVMC only supports the Fork configurations supported by ethereum/go-ethereum (eg. Byzantium, Constantinople, &c). Thus, the implementation at core-geth of EVMC requires a somewhat arbitrary mapping of granular features as keys toggling entire Ethereum fork configurations.

The following code snippet, taken from ./core/vm/evmc.go, handles this translation.

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// getRevision translates ChainConfig's HF block information into EVMC revision.
func getRevision(env *EVM) evmc.Revision {
    n := env.Context.BlockNumber
    conf := env.ChainConfig()
    switch {
    // This is an example of choosing to use an "abstracted" idea
    // about chain config, where I'm choosing to prioritize "indicative" features
    // as identifiers for Fork-Feature-Groups. Note that this is very different
    // than using Feature-complete sets to assert "did Forkage."
    case conf.IsEnabled(conf.GetEIP2565Transition, n):
        panic("berlin is unsupported by EVMCv7")
    case conf.IsEnabled(conf.GetEIP1884Transition, n):
        return evmc.Istanbul
    case conf.IsEnabled(conf.GetEIP1283DisableTransition, n):
        return evmc.Petersburg
    case conf.IsEnabled(conf.GetEIP145Transition, n):
        return evmc.Constantinople
    case conf.IsEnabled(conf.GetEIP198Transition, n):
        return evmc.Byzantium
    case conf.IsEnabled(conf.GetEIP155Transition, n):
        return evmc.SpuriousDragon
    case conf.IsEnabled(conf.GetEIP150Transition, n):
        return evmc.TangerineWhistle
    case conf.IsEnabled(conf.GetEIP7Transition, n):
        return evmc.Homestead
    default:
        return evmc.Frontier
    }
}

As you can see, individual features, like EIP1884, are translated as proxy signifiers for entire fork configurations (in this case, an Istanbul-featured VM revision). This approach, rather than requiring a complete set of the compositional features for any of these given Ethereum forks, trades a descriptive 1:1 mapping for application flexibility. Pursuing a necessarily complete feature-set -> fork map would presume chain features that are not necessarily relevant to the virtual machine, like block reward configurations, or difficulty configurations, for example. This approach allows applications to use advanced opcodes with the fewest number of incidental restrictions.

This approach is not without risk or nuance however; without a solid understanding of customizations here, experiments in customization can result in foot shooting.