Merge "Add VM preemption latency measuring tool vm-trace"

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+vm-trace is a tool utilizing the ftrace infrastructure in Linux kernel to
+measure VM preemption latencies.  For more info about ftrace, see
+Documentation/trace/ftrace.txt.  See include/linux/ring_buffer.h and
+include/linux/ftrace_event.h for data structures used by ftrace.
+
+The tool enables 2 trace points in KVM driver:
+kvm_exit defined in vmx_vcpu_run() (see arch/x86/kvm/vmx.c), and
+kvm_entry defined in vcpu_enter_guest() (see arch/x86/kvm/x86.c).
+
+It then spawns a thread to extract trace data from the kernel ftrace ring
+buffer using the splice() system call.  Once the tracing duration has elapsed,
+vm-trace calculates VM exit-entry latencies based on the timestamps of the
+events.  (A future improvement could be to spawn another thread to process the
+trace on the fly to improve vm-trace's performance.)
+
+To take a trace, do the following:
+
+1. Run qemu-kvm to start guest VM
+2. Bind each qemu-kvm vCPU thread to an isolated pCPU
+3. Start desired workload on the guest
+4. Run vm-trace on the host:
+   vm-trace -p cpu_to_trace -c cpu_to_collect_trace -s duration_in_seconds
+
+cpu_to_trace is one of the pCPUs from step 2 above that you want to trace.
+vm-trace does not support tracing multiple pCPUs.
+
+cpu_to_collect_trace is the CPU used to read and save the trace data.
+If the host system is NUMA, make sure to assign a CPU in the same NUMA node
+as cpu_to_trace to cpu_to_collect_trace.
+
+A binary file named trace.bin will be saved in the current working directory.
+Be aware that, depending on the tracing duration and type of workload running
+on the guest, the file can become quite large.
+
+vm-trace requires root privileges.
+
+Some statistics of the events will be displayed similar to the following:
+
+  Number of VM events = 21608832
+  Average VM Exit-Entry latency = 1us
+  Maximum VM Exit-Entry latency = 5us
+  Maximum cumulative latency within 1ms = 12us
+
+trace.bin will be overwritten each time vm-trace is run in this mode,
+so rename/copy the file if you want to keep it.
+
+To process a previously collected trace file, run:
+  vm-trace -f trace_file [-v]
+
+If -v is specified, all events in the trace file will be displayed.
+This is helpful for identifying cause of long latency.