ateom-microvm: upgrade cloud-hypervisor to v53 and boot directly to agent

cmd/ateom-microvm/internal/ch/guestclock.go

@@ -0,0 +1,36 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ch
+
+// clockAdvanceSince is the first cloud-hypervisor release that catches the guest
+// clock up to wall-clock time when restoring a snapshot: on aarch64 it advances
+// CNTVCT_EL0 by the downtime, and on x86 it keeps KVM_CLOCK_REALTIME set so the
+// kernel adjusts kvmclock at KVM_SET_CLOCK. Before it, a restored guest resumes
+// frozen at the instant it was snapshotted and something in the guest has to notice.
+var clockAdvanceSince = [3]int{53, 0, 0}
+
+// AdvancesGuestClockOnRestore reports whether this VMM repairs the guest clock
+// across a restore by itself.
+//
+// It reports false when the version cannot be read or parsed, which is the safe
+// direction: a caller that wrongly believes the VMM corrects the clock leaves the
+// guest reading a stale time after every resume, with no error to show for it.
+func (i VMMInfo) AdvancesGuestClockOnRestore() bool {
+	v, ok := i.semver()
+	if !ok {
+		return false
+	}
+	return compareVersions(v, clockAdvanceSince) >= 0
+}

cmd/ateom-microvm/internal/ch/guestclock_test.go

@@ -0,0 +1,50 @@
+// Copyright 2026 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package ch
+
+import "testing"
+
+func TestAdvancesGuestClockOnRestore(t *testing.T) {
+	for _, tc := range []struct {
+		name string
+		info VMMInfo
+		want bool
+	}{
+		// Real payloads, as reported by the binaries we ship. Measured: with chronyd
+		// masked and 150s of downtime, a v53 guest resumes reading the correct wall
+		// clock and a v52 guest is 160s behind.
+		{"v52 does not", VMMInfo{Version: "52.0.0", BuildVersion: "v52.0"}, false},
+		{"v53 does", VMMInfo{Version: "53.0.0", BuildVersion: "v53.0"}, true},
+		{"later releases keep the fix", VMMInfo{Version: "60.1.2"}, true},
+		{"much older", VMMInfo{Version: "41.0.0"}, false},
+
+		// The semver field is preferred, but the release tag is enough on its own.
+		{"tag only", VMMInfo{BuildVersion: "v52.0"}, false},
+		{"tag only, fixed", VMMInfo{BuildVersion: "v53.0"}, true},
+		{"suffixed", VMMInfo{Version: "53.0.0-dirty"}, true},
+
+		// Unknown means "assume it does not": believing a VMM corrects the clock when
+		// it does not leaves every resumed guest reading a stale time, silently.
+		{"empty", VMMInfo{}, false},
+		{"garbage", VMMInfo{Version: "not-a-version"}, false},
+		{"partial garbage", VMMInfo{Version: "53.x.0"}, false},
+	} {
+		t.Run(tc.name, func(t *testing.T) {
+			if got := tc.info.AdvancesGuestClockOnRestore(); got != tc.want {
+				t.Errorf("AdvancesGuestClockOnRestore() = %v, want %v", got, tc.want)
+			}
+		})
+	}
+}

cmd/ateom-microvm/run.go

@@ -117,6 +117,10 @@ const (
 	assetVirtiofsd = "virtiofsd"
 )
 
+// kataAgentPath is where the kata guest image keeps the agent binary. buildVMConfig
+// boots it as PID 1 (init=), so it is the guest's entire userspace.
+const kataAgentPath = "/usr/bin/kata-agent"
+
 // vmmMemReserveMiB is the DEFAULT guest RAM held back from the pod's memory limit
 // for the cloud-hypervisor VMM + virtiofsd, which run as host processes in the same
 // pod cgroup as the guest RAM; without a margin the pod OOMs. Overridable per
@@ -481,7 +485,8 @@ func (s *AteomService) coldBootActor(ctx context.Context, p actorBootParams) (re
 	// host-side overlay upper through the shared mount). serialLog is also read on a
 	// failed agent dial below, so keep it here.
 	serialLog := filepath.Join(kata.VMDir(actorUID), "serial.log")
-	vmCfg := buildVMConfig(actorUID, kernel, image, kparams, serialLog, memMiB, vcpus, durable)
+	vmCfg := buildVMConfig(actorUID, kernel, image, kparams, serialLog, memMiB, vcpus, durable,
+		agentInit(ctx, client.Info()))
 	if err := client.CreateVM(ctx, vmCfg); err != nil {
 		return fmt.Errorf("while creating VM: %w", err)
 	}
@@ -725,24 +730,66 @@ func (s *AteomService) guestSize(sz sizing.SandboxSize) (sizing.SandboxSize, err
 	return sz, nil
 }
 
-// buildVMConfig assembles the cloud-hypervisor VmConfig. The kernel cmdline replicates
-// kata's clh boot cmdline; beyond the base params it must set
-// systemd.unit=kata-containers.target (else the guest powers off ~6s in) and mask
-// systemd-networkd (the agent owns eth0). The console is arch-specific: ttyAMA0 on
-// arm64, ttyS0 on amd64. /dev/vda is the RO guest image; the actor rootfs's RO lower is
-// the virtio-fs device on PCI segment 1 (hence num_pci_segments=2), with no actor disks.
+// agentInit reports whether to boot the kata agent as the guest's PID 1, from what
+// the VMM just told us about itself over vmm.ping.
+//
+// Booting the agent directly skips systemd entirely, which is most of what the guest
+// reads at boot and most of what its snapshot carries. The catch is that it also drops
+// chronyd (kata-containers.target wants it), and chronyd is what repairs the guest
+// clock after a resume — so this is only safe on a VMM that advances the guest clock
+// across a restore itself. On an older or unreadable version, boot systemd instead and
+// keep the guest correct at the cost of the memory.
+func agentInit(ctx context.Context, info ch.VMMInfo) bool {
+	if info.AdvancesGuestClockOnRestore() {
+		return true
+	}
+	slog.InfoContext(ctx, "VMM does not advance the guest clock on restore; booting systemd to keep chronyd",
+		slog.String("vmm_version", info.Version), slog.String("vmm_build_version", info.BuildVersion))
+	return false
+}
+
+// initParams returns the kernel cmdline parameters that select the guest's PID 1.
+// The systemd path must name kata's target (else the guest powers off ~6s in) and
+// mask systemd-networkd, since the agent owns eth0.
+func initParams(agentInit bool) string {
+	if agentInit {
+		return "init=" + kataAgentPath
+	}
+	return "systemd.unit=kata-containers.target " +
+		"systemd.mask=systemd-networkd.service systemd.mask=systemd-networkd.socket"
+}
+
+// buildVMConfig assembles the cloud-hypervisor VmConfig. The console is arch-specific:
+// ttyAMA0 on arm64, ttyS0 on amd64. /dev/vda is the RO guest image; the actor rootfs's RO
+// lower is the virtio-fs device on PCI segment 1 (hence num_pci_segments=2), with no
+// actor disks.
+//
+// init=kataAgentPath boots the kata agent as PID 1 instead of systemd. The agent detects
+// that it is PID 1 and does the init work itself: it mounts /proc, /sys, devtmpfs /dev,
+// /dev/shm, /dev/pts, tmpfs /run and the cgroup hierarchy, then serves ttrpc over vsock.
+// Nothing else in the guest image is ours to run — the workload is a container the agent
+// starts — so systemd only cost us. Measured on the counter demo, dropping it took the
+// guest's boot-time reads from this disk from 58.6MiB to 35.0MiB, the snapshot from 145MiB
+// to 106.6MiB at the same guest RAM, and a cold boot from 15.9s to 10.3s: the agent is
+// PID 1 rather than a unit systemd reaches several seconds in, so ateom stops waiting for
+// it (the dial phase goes 10.4s -> 4.7s).
+//
+// Dropping systemd also drops chronyd (kata-containers.target wants it), which is what
+// used to repair the guest clock after a resume. That is safe only from cloud-hypervisor
+// v53, which advances the guest clock across a restore itself; on v52 a restored guest
+// stays frozen at the instant it was snapshotted.
 //
 // withDurable adds a second virtio-fs device for the actor's writable durable-dir
 // volumes (see durable.go), served by its own virtiofsd on the same PCI segment.
 // The disk-backed rootfs upper share (see rootfsupper.go) is always present.
-func buildVMConfig(id, kernel, image, kparams, serialLog string, memMiB, vcpus int, withDurable bool) ch.VmConfig {
+func buildVMConfig(id, kernel, image, kparams, serialLog string, memMiB, vcpus int, withDurable, agentInit bool) ch.VmConfig {
 	console := "ttyS0"
 	if runtime.GOARCH == "arm64" {
 		console = "ttyAMA0"
 	}
 	cmdline := "root=/dev/vda1 rootflags=data=ordered,errors=remount-ro ro rootfstype=ext4 " +
 		"panic=1 no_timer_check noreplace-smp console=" + console + ",115200n8 " +
-		"systemd.unit=kata-containers.target systemd.mask=systemd-networkd.service systemd.mask=systemd-networkd.socket"
+		initParams(agentInit)
 	if kparams != "" {
 		cmdline += " " + kparams
 	}

cmd/ateom-microvm/run_test.go

@@ -21,6 +21,7 @@ import (
 	"errors"
 	"net"
 	"path/filepath"
+	"strings"
 	"testing"
 	"time"
 
@@ -174,3 +175,26 @@ func TestGuestSize(t *testing.T) {
 		t.Errorf("guestSize(%dMiB) = %+v, nil; want an error", reserve+1, gotErr)
 	}
 }
+
+func TestInitParams(t *testing.T) {
+	// The agent path must be the one the kata guest image actually ships, since the
+	// kernel silently panics on an init= that does not exist.
+	if got := initParams(true); got != "init=/usr/bin/kata-agent" {
+		t.Errorf("initParams(true) = %q", got)
+	}
+	// Without the agent as PID 1, systemd needs kata's target — it powers the guest
+	// off within seconds otherwise — and networkd must stay masked, the agent owns eth0.
+	systemd := initParams(false)
+	for _, want := range []string{
+		"systemd.unit=kata-containers.target",
+		"systemd.mask=systemd-networkd.service",
+		"systemd.mask=systemd-networkd.socket",
+	} {
+		if !strings.Contains(systemd, want) {
+			t.Errorf("initParams(false) = %q, missing %q", systemd, want)
+		}
+	}
+	if strings.Contains(systemd, "init=") {
+		t.Errorf("initParams(false) = %q, must not override init", systemd)
+	}
+}

hack/microvm-assets/assemble.sh

@@ -34,7 +34,7 @@
 # publishes a prebuilt static binary for x86_64 only; arm64 builds from the release
 # tag, which needs rust (rustup) + libcap-ng-dev libseccomp-dev pkg-config.
 #
-# Env: ARCH (arm64|amd64, default arm64), KATA_VER (4.0.0), CH_VER (v52.0),
+# Env: ARCH (arm64|amd64, default arm64), KATA_VER (4.0.0), CH_VER (v53.0),
 #      OUT (default ./bin/microvm-assets/$ARCH, under the gitignored bin/).
 
 set -o errexit -o nounset -o pipefail
@@ -43,7 +43,7 @@ ROOT="$(git rev-parse --show-toplevel)"
 
 ARCH="${ARCH:-arm64}"
 KATA_VER="${KATA_VER:-4.0.0}"
-CH_VER="${CH_VER:-v52.0}"
+CH_VER="${CH_VER:-v53.0}"
 OUT="${OUT:-${ROOT}/bin/microvm-assets/$ARCH}"
 WORK="$(mktemp -d)"
 trap 'rm -rf "$WORK"' EXIT

manifests/microvm/sandboxconfig-microvm.yaml.tmpl

@@ -52,7 +52,7 @@ spec:
     arm64:
       cloud-hypervisor:
         url: "gs://${BUCKET_NAME}/kata-assets/cloud-hypervisor"
-        sha256: "bf004ddc1a148f47caa87ac49a783b8dbd6bf9bc27abe522ed197df7b982d3b1"
+        sha256: "f192b510eea1c710cbc439d716bb0573c223fc463dbe3e6523788a2b7ef62850"
       # virtiofsd serves the overlay RO lower (virtio-fs); v1.14.0 carries the
       # vhost-0.16 snapshot/restore fix the kata-bundled v1.13.3 lacks.
       virtiofsd:
@@ -75,7 +75,7 @@ spec:
     amd64:
       cloud-hypervisor:
         url: "gs://${BUCKET_NAME}/kata-assets/cloud-hypervisor"
-        sha256: "829af01ff075bb96c4f183905134c453a88d68cbabdc6b87df21098842581ee9"
+        sha256: "448af3d4e59b22c2987f7df94c213ad40fb53a10d437e42b5ee6c4fce7c29ecc"
       virtiofsd:
         url: "gs://${BUCKET_NAME}/kata-assets/virtiofsd"
         # The x86_64-musl binary attached to the upstream v1.14.0 release (downloaded