tools/kvmexit_example.txt

Demonstrations of kvm exit reasons, the Linux eBPF/bcc version.


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.


Features of this tool
=====================

- Although there is a patch: [KVM: x86: add full vm-exit reason debug entries]
  (https://patchwork.kernel.org/project/kvm/patch/1555939499-30854-1-git-send-email-pizhenwei@bytedance.com/)
  trying to fill more vm-exit reason debug entries, just as the comments said,
  the code allocates lots of memory that may never be consumed, misses some
  arch-specific kvm causes, and can not do kernel aggregation. Instead bcc, as
  a user space tool, can implement all these functions more easily and flexibly.
- The bcc python logic could provide nice kernel aggregation and custom output,
  like collpasing all tids for one pid (e.i. one vm's qemu process id) with exit
  reasons sorted in descending order. For more information, see the following
  #USAGE message.
- The bpf in-kernel percpu_array and percpu_cache further improves performance.
  For more information, see the following #Help to understand.


Limited
=======

In view of the hardware-assisted virtualization technology of
different architectures, currently we only adapt on vmx in intel.
And the amd feature is on the road..


Example output:
===============

# ./kvmexit.py
Display kvm exit reasons and statistics for all threads... Hit Ctrl-C to end.
PID      TID      KVM_EXIT_REASON                     COUNT
^C1273551  1273568  EXIT_REASON_HLT                     12
1273551  1273568  EXIT_REASON_MSR_WRITE               6
1274253  1274261  EXIT_REASON_EXTERNAL_INTERRUPT      1
1274253  1274261  EXIT_REASON_HLT                     12
1274253  1274261  EXIT_REASON_MSR_WRITE               4

# ./kvmexit.py 6
Display kvm exit reasons and statistics for all threads after sleeping 6 secs.
PID      TID      KVM_EXIT_REASON                     COUNT
1273903  1273922  EXIT_REASON_EXTERNAL_INTERRUPT      175
1273903  1273922  EXIT_REASON_CPUID                   10
1273903  1273922  EXIT_REASON_HLT                     6043
1273903  1273922  EXIT_REASON_IO_INSTRUCTION          24
1273903  1273922  EXIT_REASON_MSR_WRITE               15025
1273903  1273922  EXIT_REASON_PAUSE_INSTRUCTION       11
1273903  1273922  EXIT_REASON_EOI_INDUCED             12
1273903  1273922  EXIT_REASON_EPT_VIOLATION           6
1273903  1273922  EXIT_REASON_EPT_MISCONFIG           380
1273903  1273922  EXIT_REASON_PREEMPTION_TIMER        194
1273551  1273568  EXIT_REASON_EXTERNAL_INTERRUPT      18
1273551  1273568  EXIT_REASON_HLT                     989
1273551  1273568  EXIT_REASON_IO_INSTRUCTION          10
1273551  1273568  EXIT_REASON_MSR_WRITE               2205
1273551  1273568  EXIT_REASON_PAUSE_INSTRUCTION       1
1273551  1273568  EXIT_REASON_EOI_INDUCED             5
1273551  1273568  EXIT_REASON_EPT_MISCONFIG           61
1273551  1273568  EXIT_REASON_PREEMPTION_TIMER        14

# ./kvmexit.py -p 1273795 5
Display kvm exit reasons and statistics for PID 1273795 after sleeping 5 secs.
KVM_EXIT_REASON                     COUNT
MSR_WRITE                           13467
HLT                                 5060
PREEMPTION_TIMER                    345
EPT_MISCONFIG                       264
EXTERNAL_INTERRUPT                  169
EPT_VIOLATION                       18
PAUSE_INSTRUCTION                   6
IO_INSTRUCTION                      4
EOI_INDUCED                         2

# ./kvmexit.py -p 1273795 5 -a
Display kvm exit reasons and statistics for PID 1273795 and its all threads after sleeping 5 secs.
TID      KVM_EXIT_REASON                     COUNT
1273819  EXTERNAL_INTERRUPT                  64
1273819  HLT                                 2802
1273819  IO_INSTRUCTION                      4
1273819  MSR_WRITE                           7196
1273819  PAUSE_INSTRUCTION                   2
1273819  EOI_INDUCED                         2
1273819  EPT_VIOLATION                       6
1273819  EPT_MISCONFIG                       162
1273819  PREEMPTION_TIMER                    194
1273820  EXTERNAL_INTERRUPT                  78
1273820  HLT                                 2054
1273820  MSR_WRITE                           5199
1273820  EPT_VIOLATION                       2
1273820  EPT_MISCONFIG                       77
1273820  PREEMPTION_TIMER                    102

# ./kvmexit.py -p 1273795 -v 0
Display kvm exit reasons and statistics for PID 1273795 VCPU 0... Hit Ctrl-C to end.
KVM_EXIT_REASON                     COUNT
^CMSR_WRITE                           2076
HLT                                 795
PREEMPTION_TIMER                    86
EXTERNAL_INTERRUPT                  20
EPT_MISCONFIG                       10
PAUSE_INSTRUCTION                   2
IO_INSTRUCTION                      2
EPT_VIOLATION                       1
EOI_INDUCED                         1

# ./kvmexit.py -p 1273795 -v 0 4
Display kvm exit reasons and statistics for PID 1273795 VCPU 0 after sleeping 4 secs.
KVM_EXIT_REASON                     COUNT
MSR_WRITE                           4726
HLT                                 1827
PREEMPTION_TIMER                    78
EPT_MISCONFIG                       67
EXTERNAL_INTERRUPT                  28
IO_INSTRUCTION                      4
EOI_INDUCED                         2
PAUSE_INSTRUCTION                   2

# ./kvmexit.py -p 1273795 -v 4 4
Traceback (most recent call last):
  File "tools/kvmexit.py", line 306, in <module>
      raise Exception("There's no v%s for PID %d." % (tgt_vcpu, args.pid))
      Exception: There's no vCPU 4 for PID 1273795.

# ./kvmexit.py -t 1273819 10
Display kvm exit reasons and statistics for TID 1273819 after sleeping 10 secs.
KVM_EXIT_REASON                     COUNT
MSR_WRITE                           13318
HLT                                 5274
EPT_MISCONFIG                       263
PREEMPTION_TIMER                    171
EXTERNAL_INTERRUPT                  109
IO_INSTRUCTION                      8
PAUSE_INSTRUCTION                   5
EOI_INDUCED                         4
EPT_VIOLATION                       2

# ./kvmexit.py -T '1273820,1273819'
Display kvm exit reasons and statistics for TIDS ['1273820', '1273819']... Hit Ctrl-C to end.
TIDS     KVM_EXIT_REASON                     COUNT
^C1273819  EXTERNAL_INTERRUPT                  300
1273819  HLT                                 13718
1273819  IO_INSTRUCTION                      26
1273819  MSR_WRITE                           37457
1273819  PAUSE_INSTRUCTION                   13
1273819  EOI_INDUCED                         13
1273819  EPT_VIOLATION                       53
1273819  EPT_MISCONFIG                       654
1273819  PREEMPTION_TIMER                    958
1273820  EXTERNAL_INTERRUPT                  212
1273820  HLT                                 9002
1273820  MSR_WRITE                           25495
1273820  PAUSE_INSTRUCTION                   2
1273820  EPT_VIOLATION                       64
1273820  EPT_MISCONFIG                       396
1273820  PREEMPTION_TIMER                    268


Help to understand
==================

We use 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 (pcpu as follows) 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.

BTW, we originally use a common hash to do this, with a u64(exit_reason)
key and a struct exit_info {tgid_pid, exit_reason} value. But due to
the big lock in bpf_hash, each updating is quite performance consuming.

Now imagine here is a pid_tgidA (vcpu A) exits and is going to run on
pcpuArrayA, the BPF code flow is as follows:

               pid_tgidA keeps running on the same pcpu
                        //               \\
                       //                 \\
                      // Y               N \\
                     //                     \\
             a. cache_hit               b. cache_miss
(cacheA's pid_tgid matches pid_tgidA)        ||
                  |                          ||
                  |                          ||
   "increase percpu exit_ct and return"      ||
            [*Note*]                         ||
                             pid_tgidA ever been exited on pcpuArrayA?
                                           //   \\
                                          //     \\
                                         //       \\
                                        // Y     N \\
                                       //           \\
                          b.a load_last_hashA   b.b initialize_hashA_with_zero
                                          \       /
                                           \     /
                                            \   /
                                      "increase percpu exit_ct"
                                             ||
                                             ||
                           is another pid_tgid been running on pcpuArrayA?
                                        //          \\
                                       // Y        N \\
                                      //              \\
                       b.*.a save_theLastHit_hashB    do_nothing
                                           \\       //
                                            \\     //
                                             \\   //
                                       b.* save_to_pcpuArrayA


[*Note*] we do not update the table in above "a.", in case the vcpu hit the same
pcpu again when exits next time, instead we only update until this pcpu is not
hitted by the same tgidpid(vcpu) again, which is in "b.*.a" and "b.*".


USAGE message:
==============

# ./kvmexit.py -h
usage: kvmexit.py [-h] [-p PID [-v VCPU | -a] ] [-t TID | -T 'TID1,TID2'] [duration]

Display kvm_exit_reason and its statistics at a timed interval

optional arguments:
  -h, --help            show this help message and exit
  -p PID, --pid PID     display process with this PID only, collpase all tids with exit reasons sorted in descending order
  -v VCPU, --v VCPU     display this VCPU only for this PID
  -a, --alltids         display all TIDS for this PID
  -t TID, --tid TID     display thread with this TID only with exit reasons sorted in descending order
  -T 'TID1,TID2', --tids 'TID1,TID2'
                        display threads for a union like {395490, 395491}
  duration              duration of display, after sleeping several seconds

examples:
    ./kvmexit                         # Display kvm_exit_reason and its statistics in real-time until Ctrl-C
    ./kvmexit 5                       # Display in real-time after sleeping 5s
    ./kvmexit -p 3195281              # Collpase all tids for pid 3195281 with exit reasons sorted in descending order
    ./kvmexit -p 3195281 20           # Collpase all tids for pid 3195281 with exit reasons sorted in descending order, and display after sleeping 20s
    ./kvmexit -p 3195281 -v 0         # Display only vcpu0 for pid 3195281, descending sort by default
    ./kvmexit -p 3195281 -a           # Display all tids for pid 3195281
    ./kvmexit -t 395490               # Display only for tid 395490 with exit reasons sorted in descending order
    ./kvmexit -t 395490 20            # Display only for tid 395490 with exit reasons sorted in descending order after sleeping 20s
    ./kvmexit -T '395490,395491'      # Display for a union like {395490, 395491}