diff --git a/.appveyor.yml b/.appveyor.yml
index 5cb5267a6d..cf129c14d8 100644
--- a/.appveyor.yml
+++ b/.appveyor.yml
@@ -18,6 +18,8 @@ branches:
   - try
 
 install:
+# Setting build env
+- set RUST_OS_NAME=Windows
 # Compute the Rust version we use.
 - set /p RUSTC_HASH=<rust-version
 # Install Rust. We use the "stable" toolchain for better caching, it is just used to build `rustup-toolchain-install-master`.
@@ -36,7 +38,8 @@ install:
 - cargo --version
 
 test_script:
-- set PYTHON=C:\msys64\mingw64\bin\python3.exe
+# Add python3 path: https://www.appveyor.com/docs/windows-images-software/#python
+- set PATH=C:\Python35-x64;%PATH%
 - bash ci.sh
 
 after_test:
diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 101618d1a7..25ba883482 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -33,3 +33,98 @@ jobs:
             host_target: i686-pc-windows-msvc
     steps:
       - uses: actions/checkout@v2
+
+      - name: Setting build env
+        run: |
+          echo "RUST_OS_NAME=${{ runner.os }}" >> $GITHUB_ENV
+        shell: bash
+
+      # We install gnu-tar because BSD tar is buggy on Github's macos machines.
+      # See <https://github.com/actions/cache/issues/403>.
+      - name: Install GNU tar
+        if: runner.os == 'macOS'
+        run: |
+          brew install gnu-tar
+          echo "/usr/local/opt/gnu-tar/libexec/gnubin" >> $GITHUB_PATH
+
+      # Cache the global cargo directory, but NOT the local `target` directory which
+      # we cannot reuse anyway when the nightly changes (and it grows quite large
+      # over time).
+      - name: Add cache for cargo
+        uses: actions/cache@v2
+        with:
+          path: |
+            # to avoid saving rustup proxy, save xargo and RTIM in separate dir.
+            ~/.cargo/ci
+            ~/.cargo/registry
+            ~/.cargo/git
+          key: ${{ runner.os }}-cargo-${{ hashFiles('**/Cargo.lock') }}
+          restore-keys: ${{ runner.os }}-cargo
+
+      # We use the "stable" toolchain for better caching, it is just used to build `rustup-toolchain-install-master`.
+      # But we also need to take into account that the build cache might have a different, outdated default.
+      - name: Install stable Rust
+        run: |
+          # rustup toolchain uninstall beta nightly master
+          rustup override unset
+          rustup toolchain install stable
+          rustup default stable
+
+      - name: Install RTIM
+        run: |
+          cargo install --root ~/.cargo/ci --debug rustup-toolchain-install-master
+          echo "$HOME/.cargo/ci/bin" >> $GITHUB_PATH
+        shell: bash
+
+      - name: Install xargo
+        run: |
+          cargo install --root ~/.cargo/ci --debug xargo
+        shell: bash
+
+      - name: Install "master" toolchain
+        run: |
+          if [[ ${{ github.event_name }} == 'schedule' ]]; then
+            RUSTC_HASH=$(git ls-remote https://github.com/rust-lang/rust.git master | awk '{print $1}')
+          else
+            RUSTC_HASH=$(< rust-version)
+          fi
+          rustup-toolchain-install-master \
+            -f \
+            -n master "$RUSTC_HASH" \
+            -c rust-src \
+            -c rustc-dev \
+            -c llvm-tools \
+            --host ${{ matrix.host_target }}
+          rustup default master
+        shell: bash
+
+      - name: Inspect Rust version
+        run: |
+          rustup show
+          rustc -Vv
+          cargo -V
+
+      - name: Test
+        if: false
+        run: bash ./ci.sh
+
+      # Don't cache "master" toolchain, it's a waste.
+      - name: Before caching
+        run: |
+          # Binary artifacts is saved in `ci/bin` dir.
+          strip ~/.cargo/ci/bin/*
+        shell: bash
+
+  rustfmt:
+    runs-on: ubuntu-latest
+    # FIXME: the project isn't fmt-ed yet.
+    if: false
+    steps:
+      - uses: actions/checkout@v2
+      - run: rustup toolchain install stable -c rustfmt
+      - run: |
+          cargo fmt -- --check
+          # Some are not in cargo workspace.
+          git ls-files 'bench*/*.rs' | xargs rustfmt --check
+          git ls-files 'cargo-miri/*.rs' | xargs rustfmt --check
+          git ls-files 'tests*/*.rs' | xargs rustfmt --check
diff --git a/.travis.yml b/.travis.yml
index fcef17b124..7acd263b18 100644
--- a/.travis.yml
+++ b/.travis.yml
@@ -27,6 +27,19 @@ before_script:
   else
     RUSTC_HASH=$(cat rust-version)
   fi
+# Setting build env
+- |
+  case $TRAVIS_OS_NAME in
+    linux )
+      export RUST_OS_NAME=Linux
+      ;;
+    osx )
+      export RUST_OS_NAME=macOS
+      ;;
+    * )
+      echo "Doesn't know this OS"
+      exit 1;
+  esac
 # Install Rust. We use the "stable" toolchain for better caching, it is just used to build `rustup-toolchain-install-master`.
 # But we also need to take into account that the build cache might have a different, outdated default.
 - curl https://build.travis-ci.org/files/rustup-init.sh -sSf | sh -s -- -y --default-toolchain none --profile minimal
diff --git a/README.md b/README.md
index 2b01510482..418b7a1ee3 100644
--- a/README.md
+++ b/README.md
@@ -1,5 +1,11 @@
-# Miri [![Build Status](https://travis-ci.com/rust-lang/miri.svg?branch=master)](https://travis-ci.com/rust-lang/miri) [![Windows build status](https://ci.appveyor.com/api/projects/status/github/rust-lang/miri?svg=true)](https://ci.appveyor.com/project/rust-lang-libs/miri)
+# Miri
 
+[![Actions build status][actions-badge]][actions-url]
+[![Travis build status](https://travis-ci.com/rust-lang/miri.svg?branch=master)](https://travis-ci.com/rust-lang/miri)
+[![Appveyor Windows build status](https://ci.appveyor.com/api/projects/status/github/rust-lang/miri?svg=true)](https://ci.appveyor.com/project/rust-lang-libs/miri)
+
+[actions-badge]: https://github.com/rust-lang/miri/workflows/CI/badge.svg?branch=master
+[actions-url]: https://github.com/rust-lang/miri/actions
 
 An experimental interpreter for [Rust][rust]'s
 [mid-level intermediate representation][mir] (MIR).  It can run binaries and
diff --git a/ci.sh b/ci.sh
index a6daa80645..575e72ee1f 100755
--- a/ci.sh
+++ b/ci.sh
@@ -23,32 +23,43 @@ function run_tests {
   fi
 
   ./miri test --locked
-  if ! [ -n "${MIRI_TEST_TARGET+exists}" ]; then
+  if [ -z "${MIRI_TEST_TARGET+exists}" ]; then
     # Only for host architecture: tests with MIR optimizations
     #FIXME: Only testing opt level 1 due to <https://github.com/rust-lang/rust/issues/77564>.
     MIRIFLAGS="-Z mir-opt-level=1" ./miri test --locked
   fi
+
+  if command -v python3; then
+    PYTHON=python3
+  else
+    PYTHON=python
+  fi
+
   # "miri test" has built the sysroot for us, now this should pass without
   # any interactive questions.
-  ${PYTHON:-python3} test-cargo-miri/run-test.py
-
+  ${PYTHON} test-cargo-miri/run-test.py
   echo
 }
 
 # host
 run_tests
 
-if [ "${TRAVIS_OS_NAME:-}" == linux ]; then
-  MIRI_TEST_TARGET=i686-unknown-linux-gnu run_tests
-  MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
-  MIRI_TEST_TARGET=i686-pc-windows-msvc run_tests
-elif [ "${TRAVIS_OS_NAME:-}" == osx ]; then
-  MIRI_TEST_TARGET=mips64-unknown-linux-gnuabi64 run_tests # big-endian architecture
-  MIRI_TEST_TARGET=x86_64-pc-windows-msvc run_tests
-elif [ "${CI_WINDOWS:-}" == True ]; then
-  MIRI_TEST_TARGET=x86_64-unknown-linux-gnu run_tests
-  MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
-else
-  echo "FATAL: unknown CI platform"
-  exit 1
-fi
+case $RUST_OS_NAME in
+  Linux)
+    MIRI_TEST_TARGET=i686-unknown-linux-gnu run_tests
+    MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
+    MIRI_TEST_TARGET=i686-pc-windows-msvc run_tests
+    ;;
+  macOS )
+    MIRI_TEST_TARGET=mips64-unknown-linux-gnuabi64 run_tests # big-endian architecture
+    MIRI_TEST_TARGET=x86_64-pc-windows-msvc run_tests
+    ;;
+  Windows)
+    MIRI_TEST_TARGET=x86_64-unknown-linux-gnu run_tests
+    MIRI_TEST_TARGET=x86_64-apple-darwin run_tests
+    ;;
+  * )
+    echo "FATAL: unknown OS"
+    exit 1
+    ;;
+esac