Generally two options:

1. Latest and pray
2. Find someone else's (hopefully working) configuration and use the same revision as them

In reply to @alex:tunstall.xyz
Or is the issue unrelated to QEMU in that some things behave weirdly when userspace and kernelspace are on different architectures?

colemickens good news:

alex@alex-pc3 ~/r/O/n/ghc-cross-2 (ghc-cross-hadrian)> /nix/store/7kjpbn6fv6gqi9fx65ann5bqdsm3s9fi-riscv64-unknown-linux-gnu-ghc-9.10.3/bin/riscv64-unknown-linux-gnu-ghc --info | grep platform
 ,("target platform string","riscv64-unknown-linux")
 ,("Build platform","x86_64-unknown-linux")
 ,("Host platform","x86_64-unknown-linux")
 ,("Target platform","riscv64-unknown-linux")

I'm still double checking that the cross-compiler works, but I expect to PR the fix to Nixpkgs soon enough.

This alone is not enough to cross-compile cachix:

---> Starting interpreter on port 6072
Listening on port 6072
iserv-proxy-interpreter: tmp/nix/store/7kjpbn6fv6gqi9fx65ann5bqdsm3s9fi-riscv64-unknown-linux-gnu-ghc-9.10.3/lib/riscv64-unknown-linux-gnu-ghc-9.10.3/lib/riscv64-linux-ghc-9.10.3/ghc-prim-0.12.0-561f/libHSghc-prim-0.12.0-561f.a: RTS linker not implemented on riscv
iserv-proxy-interpreter: loadArchive "tmp/nix/store/7kjpbn6fv6gqi9fx65ann5bqdsm3s9fi-riscv64-unknown-linux-gnu-ghc-9.10.3/lib/riscv64-unknown-linux-gnu-ghc-9.10.3/lib/riscv64-linux-ghc-9.10.3/ghc-prim-0.12.0-561f/libHSghc-prim-0.12.0-561f.a": failed

iserv-proxy: <socket: 17>: hGetBufSome: resource vanished (Connection reset by peer)
HasCallStack backtrace:
  collectBacktraces, called at libraries/ghc-internal/src/GHC/Internal/Exception.hs:169:13 in ghc-internal:GHC.Internal.Exception
  toExceptionWithBacktrace, called at libraries/ghc-internal/src/GHC/Internal/IO.hs:260:11 in ghc-internal:GHC.Internal.IO
  throwIO, called at libraries/ghc-internal/src/GHC/Internal/IO/Exception.hs:315:19 in ghc-internal:GHC.Internal.IO.Exception
  ioException, called at libraries/ghc-internal/src/GHC/Internal/IO/Exception.hs:319:20 in ghc-internal:GHC.Internal.IO.Exception


/nix/store/vw5aaxwm2fppx1xarn7k41db5yr3rvww-iserv-wrapper-both/bin/iserv-wrapper: line 1: kill: (499) - No such process
<no location info>: error: External interpreter terminated (1)

% cat /proc/mtd                            
dev:    size   erasesize  name
mtd0: 00080000 00001000 "spl"
mtd1: 00010000 00001000 "uboot-env"
mtd2: 00400000 00001000 "uboot"
mtd3: 00a00000 00001000 "reserved-data"

Looks like the visionfive2_fw_payload.img should be flashed to /dev/mtd2 instead of /dev/mtd1