Run ARM tests natively instead of QEMU emulation

- Use ARM GitHub runners for aarch64 targets
- Run cargo test natively on ARM hardware
- Only use cross for musl targets
- This catches real SIGILL on ARM, not QEMU emulation

Author: vcjana

.github/workflows/ci.yml

@@ -335,19 +335,32 @@ jobs:
 
   # Test runtime crates on multiple architectures to catch architecture-specific issues
   # like crc-fast 1.4 SIGILL on ARM before merge
+  # Test runtime crates on multiple architectures to catch architecture-specific issues
+  # like crc-fast 1.4 SIGILL on ARM. ARM targets run natively on ARM runners to avoid
+  # QEMU emulation issues that may not accurately reproduce CPU feature detection bugs.
   test-runtime-architectures:
     name: Test Runtime (${{ matrix.target }})
-    runs-on: ubuntu-latest
+    runs-on: ${{ matrix.runner }}
     permissions:
       contents: read
     strategy:
       fail-fast: false
       matrix:
-        target:
-          - x86_64-unknown-linux-gnu
-          - x86_64-unknown-linux-musl
-          - aarch64-unknown-linux-gnu
-          - aarch64-unknown-linux-musl
+        include:
+          # x86_64 targets - use cross only for musl
+        - target: x86_64-unknown-linux-gnu
+          runner: ubuntu-latest
+          use_cross: false
+        - target: x86_64-unknown-linux-musl
+          runner: ubuntu-latest
+          use_cross: true
+          # ARM targets - run natively on ARM runners to catch real SIGILL
+        - target: aarch64-unknown-linux-gnu
+          runner: smithy_ubuntu-latest_8-core-arm
+          use_cross: false
+        - target: aarch64-unknown-linux-musl
+          runner: smithy_ubuntu-latest_8-core-arm
+          use_cross: true
     steps:
     - uses: actions/checkout@v4
       with:
@@ -359,19 +372,35 @@ jobs:
     - uses: Swatinem/rust-cache@v2
       with:
         key: ${{ matrix.target }}
-    - name: Install cross
+    - name: Install cross (if needed)
+      if: matrix.use_cross
       run: cargo install cross --locked --version 0.2.4
-    # Run tests instead of just building to catch runtime issues like SIGILL
+    # Run tests natively on ARM to catch real SIGILL, not QEMU emulation
     - name: Test smithy-rs runtime on ${{ matrix.target }}
       shell: bash
-      run: cross test --target ${{ matrix.target }} --manifest-path "rust-runtime/Cargo.toml" --workspace --exclude aws-smithy-http-server-python --exclude aws-smithy-http-server-typescript --exclude aws-smithy-experimental
+      run: |
+        if [ "${{ matrix.use_cross }}" = "true" ]; then
+          cross test --target ${{ matrix.target }} --manifest-path "rust-runtime/Cargo.toml" --workspace --exclude aws-smithy-http-server-python --exclude aws-smithy-http-server-typescript --exclude aws-smithy-experimental
+        else
+          cargo test --target ${{ matrix.target }} --manifest-path "rust-runtime/Cargo.toml" --workspace --exclude aws-smithy-http-server-python --exclude aws-smithy-http-server-typescript --exclude aws-smithy-experimental
+        fi
     - name: Test AWS runtime on ${{ matrix.target }}
       shell: bash
-      run: cross test --target ${{ matrix.target }} --manifest-path "aws/rust-runtime/Cargo.toml" --workspace
+      run: |
+        if [ "${{ matrix.use_cross }}" = "true" ]; then
+          cross test --target ${{ matrix.target }} --manifest-path "aws/rust-runtime/Cargo.toml" --workspace
+        else
+          cargo test --target ${{ matrix.target }} --manifest-path "aws/rust-runtime/Cargo.toml" --workspace
+        fi
     # Specifically test checksums crate which uses crc-fast
     - name: Test checksums on ${{ matrix.target }}
       shell: bash
-      run: cross test --target ${{ matrix.target }} --manifest-path "rust-runtime/Cargo.toml" -p aws-smithy-checksums
+      run: |
+        if [ "${{ matrix.use_cross }}" = "true" ]; then
+          cross test --target ${{ matrix.target }} --manifest-path "rust-runtime/Cargo.toml" -p aws-smithy-checksums
+        else
+          cargo test --target ${{ matrix.target }} --manifest-path "rust-runtime/Cargo.toml" -p aws-smithy-checksums
+        fi
 
   # Run the canary against generated SDKs
   #
@@ -394,10 +423,10 @@ jobs:
       fail-fast: false
       matrix:
         include:
-          - arch: x86_64
-            runner: smithy_ubuntu-latest_8-core
-          - arch: aarch64
-            runner: smithy_ubuntu-latest_8-core-arm
+        - arch: x86_64
+          runner: smithy_ubuntu-latest_8-core
+        - arch: aarch64
+          runner: smithy_ubuntu-latest_8-core-arm
     steps:
     - uses: GitHubSecurityLab/actions-permissions/monitor@v1
     - uses: actions/checkout@v4