diff --git a/.github/buildomat/README.md b/.github/buildomat/README.md
new file mode 100644
index 000000000000..45ce48135152
--- /dev/null
+++ b/.github/buildomat/README.md
@@ -0,0 +1,5 @@
+# Buildomat CI
+
+Builds on illumos use Oxide Computer's [buildomat CI](https://github.com/oxidecomputer/buildomat).
+
+Maintenance and compute resources are provided by Oxide Computer.
diff --git a/.github/buildomat/build-and-test.sh b/.github/buildomat/build-and-test.sh
new file mode 100755
index 000000000000..4e04b96f152d
--- /dev/null
+++ b/.github/buildomat/build-and-test.sh
@@ -0,0 +1,18 @@
+#!/bin/env bash
+
+set -o errexit
+set -o pipefail
+set -o xtrace
+
+export TARGET="$1"
+
+# Enable ANSI colors in Cargo output.
+export CARGO_TERM_COLOR=always
+
+banner install
+# Note: we use ci/install-rust.sh rather than buildomat to install Rust, for
+# consistency with other CI jobs.
+TOOLCHAIN=stable INSTALL_RUSTUP=1 ptime -m sh ci/install-rust.sh
+
+banner run.sh
+ptime -m sh ci/run.sh "$TARGET"
diff --git a/.github/buildomat/config.toml b/.github/buildomat/config.toml
new file mode 100644
index 000000000000..38356adbaff4
--- /dev/null
+++ b/.github/buildomat/config.toml
@@ -0,0 +1,7 @@
+# Configuration for buildomat. See
+# https://github.com/oxidecomputer/buildomat/blob/main/README.md#per-repository-configuration.
+
+enable = true
+# The buildomat jobs don't have access to any secrets, so it's okay for GitHub
+# users to create them.
+org_only = false
diff --git a/.github/buildomat/jobs/x86_64-unknown-illumos.sh b/.github/buildomat/jobs/x86_64-unknown-illumos.sh
new file mode 100644
index 000000000000..aa16abf37172
--- /dev/null
+++ b/.github/buildomat/jobs/x86_64-unknown-illumos.sh
@@ -0,0 +1,7 @@
+#!/bin/env bash
+#:
+#: name = "x86_64-unknown-illumos"
+#: variety = "basic"
+#: target = "helios-latest"
+
+exec .github/buildomat/build-and-test.sh x86_64-unknown-illumos
diff --git a/ci/install-rust.sh b/ci/install-rust.sh
index 16fd0b4e8a57..f28a0704b8b1 100755
--- a/ci/install-rust.sh
+++ b/ci/install-rust.sh
@@ -7,17 +7,56 @@ echo "Setup toolchain"
 
 toolchain="${TOOLCHAIN:-nightly}"
 os="${OS:-}"
+install_rustup="${INSTALL_RUSTUP:-0}"
 
 case "$(uname -s)" in
     Linux*)     os=linux ;;
     Darwin*)    os=macos ;;
     MINGW*)     os=windows ;;
+    # This captures both Solaris and illumos, which aren't possible to
+    # distinguish via uname -s. But at the moment we don't need to make this
+    # distinction -- the only distinction we care about is in TARGET, which is
+    # expected to be set in the environment.
+    SunOS*)     os=solarish ;;
     *)
         echo "Unknown system $(uname -s)"
         exit 1
         ;;
 esac
 
+if [ "$install_rustup" = "1" ]; then
+    echo "Install rustup"
+
+    # If the CI system already has Rust installed, we'll override that
+    # installation via sourcing ~/.cargo/env.
+    export RUSTUP_INIT_SKIP_PATH_CHECK=yes
+    curl https://sh.rustup.rs -sSf | sh -s -- -y --default-toolchain none
+    # shellcheck source=/dev/null
+    . "$HOME/.cargo/env"
+
+    # It is possible that "$HOME/.cargo/bin" was already in the PATH, in which
+    # case the above source would not have any effect. If the directory wasn't
+    # present on disk, then some shells negatively cache the PATH lookup and not
+    # find the directory even after it is created. To work around this, force a
+    # change to the PATH.
+    #
+    # This is a more portable version of `hash -r`. `hash -r` is part of the POSIX
+    # spec [1] but may not be available in all shells. The manual suggests the
+    # following, more portable option:
+    #
+    #    PATH="$PATH"
+    #
+    # But empirically, that has been observed to not invalidate the cache in some
+    # shells. Actually making a change to the PATH should always work (hopefully!)
+    #
+    # [1] https://pubs.opengroup.org/onlinepubs/9799919799/utilities/hash.html
+
+    # First, add a trailing colon.
+    PATH="$PATH:"
+    # Then, remove it.
+    PATH="${PATH%:}"
+fi
+
 if [ "$os" = "windows" ] && [ -n "${TARGET:-}" ]; then
     toolchain="$toolchain-$TARGET"
     rustup set profile minimal
diff --git a/libc-test/build.rs b/libc-test/build.rs
index 1173f665fb56..a49c058ffb18 100644
--- a/libc-test/build.rs
+++ b/libc-test/build.rs
@@ -966,6 +966,13 @@ fn test_solarish(target: &str) {
         // This evaluates to a sysconf() call rather than a constant
         "PTHREAD_STACK_MIN" => true,
 
+        // Note: on illumos, the value of PTHREAD_MUTEX_DEFAULT was changed to a
+        // new value, 0x8, in 2024-02:
+        // https://github.com/illumos/illumos-gate/commit/50718d3ece2504ebcfdc3f385132f664b567cdd0.
+        // libc still has the old value (= PTHREAD_MUTEX_NORMAL = 0x0) for a
+        // window of time in case illumos systems are out of date.
+        "PTHREAD_MUTEX_DEFAULT" if is_illumos => true,
+
         // EPOLLEXCLUSIVE is a relatively recent addition to the epoll interface and may not be
         // defined on older systems.  It is, however, safe to use on systems which do not
         // explicitly support it. (A no-op is an acceptable implementation of EPOLLEXCLUSIVE.)
diff --git a/src/unix/solarish/mod.rs b/src/unix/solarish/mod.rs
index 0b223cd982cf..eeac9ab0c41a 100644
--- a/src/unix/solarish/mod.rs
+++ b/src/unix/solarish/mod.rs
@@ -2063,6 +2063,15 @@ pub const PTHREAD_RWLOCK_INITIALIZER: pthread_rwlock_t = pthread_rwlock_t {
 pub const PTHREAD_MUTEX_NORMAL: c_int = 0;
 pub const PTHREAD_MUTEX_ERRORCHECK: c_int = 2;
 pub const PTHREAD_MUTEX_RECURSIVE: c_int = 4;
+
+// Note: on illumos, the value of PTHREAD_MUTEX_DEFAULT was changed to a new
+// value, 0x8, in 2024-02:
+// https://github.com/illumos/illumos-gate/commit/50718d3ece2504ebcfdc3f385132f664b567cdd0.
+// libc still has the old value (= PTHREAD_MUTEX_NORMAL = 0x0) for a window of
+// time in case illumos systems are out of date.
+//
+// Once this constant has been updated on illumos, the corresponding
+// `cfg.skip_const` configuration in `libc-test/build.rs` should be removed.
 pub const PTHREAD_MUTEX_DEFAULT: c_int = crate::PTHREAD_MUTEX_NORMAL;
 
 pub const RTLD_NEXT: *mut c_void = -1isize as *mut c_void;