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kvmexit(8) System Manager's Manual kvmexit(8)


kvmexit - Display the exit_reason and its statistics of each vm exit.


kvmexit [-h] [-p PID [-v VCPU | -a] ] [-t TID | -T 'TID1,TID2'] [duration]


Considering virtual machines' frequent exits can cause performance problems, this tool aims to locate the frequent exited reasons and then find solutions to reduce or even avoid the exit, by displaying the detail exit reasons and the counts of each vm exit for all vms running on one physical machine.

This tool uses a PERCPU_ARRAY: pcpuArrayA and a percpu_hash: hashA to collaboratively store each kvm exit reason and its count. The reason is there exists a rule when one vcpu exits and re-enters, it tends to continue to run on the same physical cpu as the last cycle, which is also called 'cache hit'. Thus we turn to use a PERCPU_ARRAY to record the 'cache hit' situation to speed things up; and for other cases, then use a percpu_hash.

As RAW_TRACEPOINT_PROBE(kvm_exit) consumes less cpu cycles, when this tool is used, it firstly tries to employ raw tracepoints in modules, and if failes, then fall back to regular tracepoint.

Limitation: In view of the hardware-assisted virtualization technology of different architectures, currently we only adapt on vmx in intel.

Since this uses BPF, only the root user can use this tool.


CONFIG_BPF and bcc.

This also requires Linux 4.7+ (BPF_PROG_TYPE_TRACEPOINT support).


Print usage message.
Display process with this PID only, collpase all tids with exit reasons sorted in descending order.
Display this VCPU only for this PID.
Display all TIDS for this PID.
Display thread with this TID only with exit reasons sorted in descending order.
Display threads for a union like {395490, 395491}.
Duration of display, after sleeping several seconds.


# kvmexit
# kvmexit 6
# kvmexit -p 1273795 5
# kvmexit -p 1273795 5 -a
# kvmexit -p 1273795 -v 0
# kvmexit -p 1273795 -v 0 4
# kvmexit -t 1273819 10
# kvmexit -T '1273820,1273819'


This traces the "kvm_exit" kernel function, records the exit reason and calculates its counts. Contrast with filling more vm-exit reason debug entries, this tool is more easily and flexibly: the bcc python logic could provide nice kernel aggregation and custom output, the bpf in-kernel percpu_array and percpu_cache further improves performance.

The impact of using this tool on the host should be negligible. While this tool is very efficient, it does affect the guest virtual machine itself, the average test results on guest vm are as follows:
| cpu cycles
no TP | 1127
regular TP | 1277 (13% downgrade)
RAW TP | 1187 (5% downgrade)

Host: echo 1 > /proc/sys/net/core/bpf_jit_enable


This is from bcc.

Also look in the bcc distribution for a companion _examples.txt file containing example usage, output, and commentary for this tool.




Unstable - in development.


Fei Li <>

2021-07-08 USER COMMANDS