Confused about cross-compiling

[rileym-arcules]

I'm trying to cross-compile my project to from host linux-x86_64 to target linux-aarch64.

My understanding from the examples/test MODULE.bazel and README is that all I should need to do is provide a sysroot for the linux-aarch64 target, but that doesn't work? I created each sysroot manually using docker export after installing clang-16 in a corresponding ubuntu:22.04 container.

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")

LLVM_VERSION = "16.0.4"

HOSTS = ["linux-x86_64"]
TARGETS = ["linux-x86_64", "linux-aarch64"]

llvm.toolchain(
    name = "llvm_toolchain",
    llvm_version = LLVM_VERSION,
)

llvm.sysroot(
    name = "llvm_toolchain",
    label = "//:linux-amd64-sysroot-bazel.tar.gz",
    targets = ["linux-x86_64"],
)

llvm.sysroot(
    name = "llvm_toolchain",
    label = "//:linux-arm64-sysroot-bazel.tar.gz",
    targets = ["linux-aarch64"],
)

use_repo(llvm, "llvm_toolchain")
register_toolchains("@llvm_toolchain//:all")

build output:

$ bazel build --toolchain_resolution_debug=@@bazel_tools//tools/cpp:toolchain_type --platforms=@toolchains_llvm//platforms:linux-aarch64 :workspace
INFO: Invocation ID: 71c45898-9f4b-47b6-bded-a4dcd9fb13d1
INFO: ToolchainResolution: Performing resolution of @@bazel_tools//tools/cpp:toolchain_type for target platform @@toolchains_llvm+//platforms:linux-aarch64
      ToolchainResolution:   Rejected toolchain @@toolchains_llvm++llvm+llvm_toolchain//:cc-clang-riscv32-none; mismatching values: riscv32, none
      ToolchainResolution:   Rejected toolchain @@toolchains_llvm++llvm+llvm_toolchain//:cc-clang-x86_64-linux; mismatching values: x86_64
      ToolchainResolution:   Rejected toolchain @@toolchains_llvm++llvm+llvm_toolchain//:cc-clang-x86_64-none; mismatching values: x86_64, none
      ToolchainResolution:   Rejected toolchain @@rules_cc++cc_configure_extension+local_config_cc//:cc-compiler-armeabi-v7a; mismatching values: armv7, android
      ToolchainResolution:   Rejected toolchain @@rules_cc++cc_configure_extension+local_config_cc//:cc-compiler-k8; mismatching values: x86_64
      // rejected apple_support toolchains
      ToolchainResolution: No @@bazel_tools//tools/cpp:toolchain_type toolchain found for target platform @@toolchains_llvm+//platforms:linux-aarch64.
ERROR: /home/rileym/.cache/bazel/_bazel_rileym/567c132ed103c5789b82d69c88fbdf82/external/rules_cc+/cc/compiler/BUILD:50:14: While resolving toolchains for target @@rules_cc+//cc/compiler:compiler (7ea8d52): No matching toolchains found for types:
  @@bazel_tools//tools/cpp:toolchain_type
To debug, rerun with --toolchain_resolution_debug='@@bazel_tools//tools/cpp:toolchain_type'
For more information on platforms or toolchains see https://bazel.build/concepts/platforms-intro.
ERROR: /home/rileym/.cache/bazel/_bazel_rileym/567c132ed103c5789b82d69c88fbdf82/external/gperftools+/BUILD.bazel:223:11: errors encountered resolving select() keys for @@gperftools+//:tcmalloc
ERROR: Analysis of target '//:workspace' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.263s, Critical Path: 0.04s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

So I then tried individual toolchains for the different platforms,

llvm = use_extension("@toolchains_llvm//toolchain/extensions:llvm.bzl", "llvm")

LLVM_VERSION = "16.0.4"

HOSTS = ["linux-x86_64"]
TARGETS = ["linux-x86_64", "linux-aarch64"]

llvm.toolchain(
    name = "llvm_toolchain-linux-amd64",
    llvm_version = LLVM_VERSION,
    exec_os = "linux",
    exec_arch = "x86_64",
)

llvm.sysroot(
    name = "llvm_toolchain-linux-amd64",
    label = "//:linux-amd64-sysroot-bazel.tar.gz",
    targets = ["linux-x86_64"],
)

use_repo(llvm, "llvm_toolchain-linux-amd64")
register_toolchains("@llvm_toolchain-linux-amd64//:cc-toolchain-x86_64-linux")

llvm.toolchain(
    name = "llvm_toolchain-linux-arm64",
    llvm_version = LLVM_VERSION,
    exec_os = "linux",
    exec_arch = "aarch64",
)

llvm.sysroot(
    name = "llvm_toolchain-linux-arm64",
    label = "//:linux-arm64-sysroot-bazel.tar.gz",
    targets = ["linux-aarch64"],
)

use_repo(llvm, "llvm_toolchain-linux-arm64")
register_toolchains("@llvm_toolchain-linux-arm64//:cc-toolchain-aarch64-linux")

which seems to get me a little further but still no build.
I don't know why the exec platform isn't resolved to the host platform?

build output:

$ bazel build --toolchain_resolution_debug=@@bazel_tools//tools/cpp:toolchain_type --platforms=@toolchains_llvm//platforms:linux-aarch64 :workspace
INFO: Invocation ID: f20d517c-9b30-4701-8cf1-a52037725227
INFO: ToolchainResolution: Performing resolution of @@bazel_tools//tools/cpp:toolchain_type for target platform @@toolchains_llvm+//platforms:linux-aarch64
      ToolchainResolution:   Rejected toolchain @@toolchains_llvm++llvm+llvm_toolchain-linux-amd64//:cc-clang-x86_64-linux; mismatching values: x86_64
      ToolchainResolution:   Toolchain @@toolchains_llvm++llvm+llvm_toolchain-linux-arm64//:cc-clang-aarch64-linux is compatible with target platform, searching for execution platforms:
      ToolchainResolution:     Incompatible execution platform @@platforms//host:host; mismatching values: aarch64
      ToolchainResolution:   Rejected toolchain @@rules_cc++cc_configure_extension+local_config_cc//:cc-compiler-armeabi-v7a; mismatching values: armv7, android
      ToolchainResolution:   Rejected toolchain @@rules_cc++cc_configure_extension+local_config_cc//:cc-compiler-k8; mismatching values: x86_64
      // rejected apple_support toolchains
      ToolchainResolution: No @@bazel_tools//tools/cpp:toolchain_type toolchain found for target platform @@toolchains_llvm+//platforms:linux-aarch64.
INFO: ToolchainResolution: Target platform @@toolchains_llvm+//platforms:linux-aarch64: Selected execution platform @@platforms//host:host, 
ERROR: /home/rileym/.cache/bazel/_bazel_rileym/567c132ed103c5789b82d69c88fbdf82/external/rules_cc+/cc/BUILD:146:19: in cc_toolchain_alias rule @@rules_cc+//cc:current_cc_toolchain: 
Traceback (most recent call last):
        File "/virtual_builtins_bzl/common/cc/cc_toolchain_alias.bzl", line 26, column 48, in _impl
        File "/virtual_builtins_bzl/common/cc/cc_helper.bzl", line 165, column 13, in _find_cpp_toolchain
Error in fail: Unable to find a CC toolchain using toolchain resolution. Target: @@rules_cc+//cc:current_cc_toolchain, Platform: @@toolchains_llvm+//platforms:linux-aarch64, Exec platform: @@bazel_tools//tools:host_platform
ERROR: /home/rileym/.cache/bazel/_bazel_rileym/567c132ed103c5789b82d69c88fbdf82/external/rules_cc+/cc/BUILD:146:19: Analysis of target '@@rules_cc+//cc:current_cc_toolchain' (config: 7ea8d52) failed
ERROR: Analysis of target '//:workspace' failed; build aborted: Analysis failed
INFO: Elapsed time: 0.336s, Critical Path: 0.04s
INFO: 1 process: 1 internal.
ERROR: Build did NOT complete successfully

How is this supposed to work?