L synthesize last branch entries on existing event records
s skip initial number of events
q quicker (less detailed) decoding
+ Z prefer to ignore timestamps (so-called "timeless" decoding)
"Instructions" events look like they were recorded by "perf record -e
instructions".
- instruction pointer associated with PSB packets
+The Z option is equivalent to having recorded a trace without TSC
+(i.e. config term tsc=0). It can be useful to avoid timestamp issues when
+decoding a trace of a virtual machine.
+
dump option
~~~~~~~~~~~
Tracing Virtual Machines
------------------------
-Currently, only kernel tracing is supported and only with "timeless" decoding
-i.e. no TSC timestamps
+Currently, only kernel tracing is supported and only with either "timeless" decoding
+(i.e. no TSC timestamps) or VM Time Correlation. VM Time Correlation is an extra step
+using 'perf inject' and requires unchanging VMX TSC Offset and no VMX TSC Scaling.
Other limitations and caveats
Guest VCPU is unknown but may be able to be inferred from the host thread
Callchains are not supported
-Example
+Example using "timeless" decoding
Start VM
:1440 1440 ffffffffbb74603c clockevents_program_event+0x4c ([guest.kernel.kallsyms]) popq %rbx
:1440 1440 ffffffffbb74603d clockevents_program_event+0x4d ([guest.kernel.kallsyms]) popq %r12
+Example using VM Time Correlation
+
+Start VM
+
+ $ sudo virsh start kubuntu20.04
+ Domain kubuntu20.04 started
+
+Mount the guest file system. Note sshfs needs -o direct_io to enable reading of proc files. root access is needed to read /proc/kcore.
+
+ $ mkdir -p vm0
+ $ sshfs -o direct_io root@vm0:/ vm0
+
+Copy the guest /proc/kallsyms, /proc/modules and /proc/kcore
+
+ $ perf buildid-cache -v --kcore vm0/proc/kcore
+ same kcore found in /home/user/.debug/[kernel.kcore]/cc9c55a98c5e4ec0aeda69302554aabed5cd6491/2021021312450777
+ $ KALLSYMS=/home/user/.debug/\[kernel.kcore\]/cc9c55a98c5e4ec0aeda69302554aabed5cd6491/2021021312450777/kallsyms
+
+Find the VM process
+
+ $ ps -eLl | grep 'KVM\|PID'
+ F S UID PID PPID LWP C PRI NI ADDR SZ WCHAN TTY TIME CMD
+ 3 S 64055 16998 1 17005 13 80 0 - 1818189 - ? 00:00:16 CPU 0/KVM
+ 3 S 64055 16998 1 17006 4 80 0 - 1818189 - ? 00:00:05 CPU 1/KVM
+ 3 S 64055 16998 1 17007 3 80 0 - 1818189 - ? 00:00:04 CPU 2/KVM
+ 3 S 64055 16998 1 17008 4 80 0 - 1818189 - ? 00:00:05 CPU 3/KVM
+
+Start an open-ended perf record, tracing the VM process, do something on the VM, and then ctrl-C to stop.
+IPC can be determined, hence cyc=1 can be added.
+Only kernel decoding is supported, so 'k' must be specified.
+Intel PT traces both the host and the guest so --guest and --host need to be specified.
+
+ $ sudo perf kvm --guest --host --guestkallsyms $KALLSYMS record --kcore -e intel_pt/cyc=1/k -p 16998
+ ^C[ perf record: Woken up 1 times to write data ]
+ [ perf record: Captured and wrote 9.041 MB perf.data.kvm ]
+
+Now 'perf inject' can be used to determine the VMX TCS Offset. Note, Intel PT TSC packets are
+only 7-bytes, so the TSC Offset might differ from the actual value in the 8th byte. That will
+have no effect i.e. the resulting timestamps will be correct anyway.
+
+ $ perf inject -i perf.data.kvm --vm-time-correlation=dry-run
+ ERROR: Unknown TSC Offset for VMCS 0x1bff6a
+ VMCS: 0x1bff6a TSC Offset 0xffffe42722c64c41
+ ERROR: Unknown TSC Offset for VMCS 0x1cbc08
+ VMCS: 0x1cbc08 TSC Offset 0xffffe42722c64c41
+ ERROR: Unknown TSC Offset for VMCS 0x1c3ce8
+ VMCS: 0x1c3ce8 TSC Offset 0xffffe42722c64c41
+ ERROR: Unknown TSC Offset for VMCS 0x1cbce9
+ VMCS: 0x1cbce9 TSC Offset 0xffffe42722c64c41
+
+Each virtual CPU has a different Virtual Machine Control Structure (VMCS)
+shown above with the calculated TSC Offset. For an unchanging TSC Offset
+they should all be the same for the same virtual machine.
+
+Now that the TSC Offset is known, it can be provided to 'perf inject'
+
+ $ perf inject -i perf.data.kvm --vm-time-correlation="dry-run 0xffffe42722c64c41"
+
+Note the options for 'perf inject' --vm-time-correlation are:
+
+ [ dry-run ] [ <TSC Offset> [ : <VMCS> [ , <VMCS> ]... ] ]...
+
+So it is possible to specify different TSC Offsets for different VMCS.
+The option "dry-run" will cause the file to be processed but without updating it.
+Note it is also possible to get a intel_pt.log file by adding option --itrace=d
+
+There were no errors so, do it for real
+
+ $ perf inject -i perf.data.kvm --vm-time-correlation=0xffffe42722c64c41 --force
+
+'perf script' can be used to see if there are any decoder errors
+
+ $ perf script -i perf.data.kvm --guestkallsyms $KALLSYMS --itrace=e-o
+
+There were none.
+
+'perf script' can be used to provide an instruction trace showing timestamps
+
+ $ perf script -i perf.data.kvm --guestkallsyms $KALLSYMS --insn-trace --xed -F+ipc | grep -C10 vmresume | head -21
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cdd __vmx_vcpu_run+0x3d ([kernel.kallsyms]) movq 0x48(%rax), %r9
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce1 __vmx_vcpu_run+0x41 ([kernel.kallsyms]) movq 0x50(%rax), %r10
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce5 __vmx_vcpu_run+0x45 ([kernel.kallsyms]) movq 0x58(%rax), %r11
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ce9 __vmx_vcpu_run+0x49 ([kernel.kallsyms]) movq 0x60(%rax), %r12
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133ced __vmx_vcpu_run+0x4d ([kernel.kallsyms]) movq 0x68(%rax), %r13
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf1 __vmx_vcpu_run+0x51 ([kernel.kallsyms]) movq 0x70(%rax), %r14
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf5 __vmx_vcpu_run+0x55 ([kernel.kallsyms]) movq 0x78(%rax), %r15
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cf9 __vmx_vcpu_run+0x59 ([kernel.kallsyms]) movq (%rax), %rax
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133cfc __vmx_vcpu_run+0x5c ([kernel.kallsyms]) callq 0xffffffff82133c40
+ CPU 1/KVM 17006 [001] 11500.262865593: ffffffff82133c40 vmx_vmenter+0x0 ([kernel.kallsyms]) jz 0xffffffff82133c46
+ CPU 1/KVM 17006 [001] 11500.262866075: ffffffff82133c42 vmx_vmenter+0x2 ([kernel.kallsyms]) vmresume IPC: 0.05 (40/769)
+ :17006 17006 [001] 11500.262869216: ffffffff82200cb0 asm_sysvec_apic_timer_interrupt+0x0 ([guest.kernel.kallsyms]) clac
+ :17006 17006 [001] 11500.262869216: ffffffff82200cb3 asm_sysvec_apic_timer_interrupt+0x3 ([guest.kernel.kallsyms]) pushq $0xffffffffffffffff
+ :17006 17006 [001] 11500.262869216: ffffffff82200cb5 asm_sysvec_apic_timer_interrupt+0x5 ([guest.kernel.kallsyms]) callq 0xffffffff82201160
+ :17006 17006 [001] 11500.262869216: ffffffff82201160 error_entry+0x0 ([guest.kernel.kallsyms]) cld
+ :17006 17006 [001] 11500.262869216: ffffffff82201161 error_entry+0x1 ([guest.kernel.kallsyms]) pushq %rsi
+ :17006 17006 [001] 11500.262869216: ffffffff82201162 error_entry+0x2 ([guest.kernel.kallsyms]) movq 0x8(%rsp), %rsi
+ :17006 17006 [001] 11500.262869216: ffffffff82201167 error_entry+0x7 ([guest.kernel.kallsyms]) movq %rdi, 0x8(%rsp)
+ :17006 17006 [001] 11500.262869216: ffffffff8220116c error_entry+0xc ([guest.kernel.kallsyms]) pushq %rdx
+ :17006 17006 [001] 11500.262869216: ffffffff8220116d error_entry+0xd ([guest.kernel.kallsyms]) pushq %rcx
+ :17006 17006 [001] 11500.262869216: ffffffff8220116e error_entry+0xe ([guest.kernel.kallsyms]) pushq %rax
+
SEE ALSO
projects / linux-2.6-microblaze.git / blobdiff