Hypervisor & guest tracing

Hypervisor tracing

Starting with android14-5.15 kernel it is possible to get traces from the hypervisor.

User space interface

The user space hypervisor tracing interface is located either at /sys/kernel/tracing/hyp or at /sys/kernel/debug/tracing/hyp. On the Android phones it will usually be /sys/kernel/tracing/hyp, while on QEMU it will be /sys/kernel/debug/tracing/hyp.

The user space interface is very similar to the ftrace user space interface, however there are some differences, e.g.:

  • Only boot clock is supported, and there is no way for user space to change the tracing_clock.
  • Hypervisor tracing periodically polls the data from the hypervisor, this is different from the regular ftrace instance which pushes the events into the ring buffer.
  • Resetting ring buffers (by clearing the trace file) is only supported when there are no active readers. If the trace file is cleared while there are active readers, then the ring buffers will be cleared after the last reader disconnects.
  • Changing the size of the ring buffer while the tracing session is active is also not supported.

Note: the list above is not exhaustive.

Perfetto integration

Perfetto is an open-source stack for performance instrumentation and trace analysis widely used in Android. Perfetto supports capturing and visualizing hypervisor traces.

Capturing hypervisor traces on Android

Consider first familiarizing yourself with Perfetto documentation for recording traces on Android: https://perfetto.dev/docs/quickstart/android-tracing.

The record_android_trace script supports a shortcut to capture all hypervisor events that are known to Perfetto:

external/perfetto/tools/record_android_trace hyp -t 15s -b 32mb -o /tmp/hyp.pftrace

Alternatively you can use full trace config to capture hypervisor. Example usage:

cat<<EOF>config.pbtx
duration_ms: 10000

buffers: {
    size_kb: 8960
    fill_policy: DISCARD
}

data_sources: {
    config {
        name: "linux.ftrace"
        ftrace_config {
            instance_name: "hyp"
            ftrace_events: "hyp/hyp_enter"
            ftrace_events: "hyp/hyp_exit"
        }
    }
}
EOF

./record_android_trace -c config.pbtx -o trace_file.perfetto-trace

If you have an Android tree checked out, then record_android_trace helper script can be located at ${REPO_ROOT}/external/perfetto/tools/record_android_traces. Otherwise, you can download the script by following steps outlined in the Perfetto docs

Capturing hypervisor traces on QEMU

Perfetto supports capturing traces on Linux: https://perfetto.dev/docs/quickstart/linux-tracing. However, since pKVM hypervisor is only supported on arm64, you will need to cross-compile Perfetto binaries for linux-arm64 (unless you have an arm64 workstation).

  1. Checkout Perfetto repository: https://perfetto.dev/docs/contributing/getting-started
  2. Follow https://perfetto.dev/docs/contributing/build-instructions#cross-compiling-for-linux-arm-64 to compile Perfetto binaries for arm64 architecture.
  3. Copy the tracebox binary to QEMU
  4. Run tracebox binary on QEMU to capture traces, it's interface is very similar to the record_android_trace binary. E.g. to capture all hypervisor events run:
tracebox -t 15s -b 32mb hyp

Analysing traces using SQL

On top of visualisation, Perfetto also provides a SQL interface to analyse traces. More documentation is available at https://perfetto.dev/docs/quickstart/trace-analysis and https://perfetto.dev/docs/analysis/trace-processor.

Hypervisor events can be queried via pkvm_hypervisor_events SQL view. You can load that view by calling SELECT IMPORT("pkvm.hypervisor");, e.g.:

SELECT IMPORT("pkvm.hypervisor");
SELECT * FROM pkvm_hypervisor_events limit 5;

Below are some SQL queries that might be useful when analysing hypervisor traces.

What is the longest time CPU spent in hypervisor, grouped by the reason to enter hypervisor

SELECT IMPORT("pkvm.hypervisor");

SELECT
  cpu,
  reason,
  ts,
  dur
FROM pkvm_hypervisor_events
JOIN (
  SELECT
    MAX(dur) as dur2,
    cpu as cpu2,
    reason as reason2
  FROM pkvm_hypervisor_events
  GROUP BY 2, 3) AS sc
ON
  cpu = sc.cpu2
  AND dur = sc.dur2
  AND (reason = sc.reason2 OR (reason IS NULL AND sc.reason2 IS NULL))
ORDER BY dur desc;

What are the 10 longest times CPU spent in hypervisor because of host_mem_abort

SELECT
  hyp.dur as dur,
  hyp.ts as ts,
  EXTRACT_ARG(slices.arg_set_id, 'esr') as esr,
  EXTRACT_ARG(slices.arg_set_id, 'addr') as addr
FROM pkvm_hypervisor_events as hyp
JOIN slices
ON hyp.slice_id = slices.id
WHERE hyp.reason = 'host_mem_abort'
ORDER BY dur desc
LIMIT 10;

Microdroid VM tracing

IMPORTANT: Tracing is only supported for debuggable Microdroid VMs.

Capturing trace in Microdroid

Starting with Android U, Microdroid contains Perfetto tracing binaries, which makes it possible to capture traces inside Microdroid VM using Perfetto stack. The commands used to capture traces on Android should work for Microdroid VM as well, with a difference that Perfetto's tracing binaries are not enabled in Microdroid by default, so you need to manually start them by setting persist.traced.enable system property to 1.

Here is a quick example on how trace Microdroid VM:

  1. First start your VM. For this example we are going to use adb shell /apex/com.android.virt/bin/vm run-microdroid.

  2. Set up an adb connection with the running VM:

adb shell forward tcp:9876 vsock:${CID}:5555
adb connect localhost:9876
adb -s localhost:9876 root

Where ${CID} corresponds to the running Microdroid VM that you want to establish adb connection with. List of running VMs can be obtained by running adb shell /apex/com.android.virt/bin/vm list. Alternatively you can use vm_shell utility to connect to a running VM, i.e.: vm_shell connect.

  1. Start Perfetto daemons and capture trace
adb -s localhost:9876 shell setprop persist.traced.enable 1
${ANDROID_BULD_TOP}/external/perfetto/tools/record_android_trace \
  -s localhost:9876 \
  -o /tmp/microdroid-trace-file.pftrace \
  -t 10s \
  -b 32mb \
  sched/sched_switch task/task_newtask sched/sched_process_exit

If you don't have Android repo checked out, then you can download the record_android_trace script by following the following instructions

More documentation on Perfetto's tracing on Android is available here: https://perfetto.dev/docs/quickstart/android-tracing

Capturing Microdroid boot trace

TODO(b/271412868): Stay tuned, more docs are coming soon!