[intel-pt] Disable/Enable tracing to guarantee the trace is correct

As mentioned in the comment inside the code, the Intel documentation
states that the internal CPU buffer is flushed out to RAM only when tracing is
disabled. Otherwise, the buffer on RAM might be stale.

This diff disables tracing when the trace buffer is going to be read. This is a
quite safe operation, as the reading is done when the inferior is paused at a
breakpoint, so we are not losing any packets because there's no code being
executed.

After the reading is finished, tracing is enabled back.

It's a bit hard to write a test for this now, but Greg Clayton and I will
refactor the PT support and writing tests for it will be easier. However
I tested it manually by doing a script that automates
the following flow

```
(lldb) b main
Breakpoint 1: where = a.out`main + 15 at main.cpp:4:7, address = 0x000000000040050f
(lldb) r
Process 3078226 stopped
* thread #1, name = 'a.out', stop reason = breakpoint 1.1
frame #0: 0x000000000040050f a.out`main at main.cpp:4:7
(lldb) processor-trace start
(lldb) b 5
Breakpoint 2: where = a.out`main + 22 at main.cpp:5:12, address = 0x0000000000400516
(lldb) c
Process 3078226 resuming
Process 3078226 stopped
* thread #1, name = 'a.out', stop reason = breakpoint 2.1
frame #0: 0x0000000000400516 a.out`main at main.cpp:5:12
(lldb) processor-trace show-instr-log
thread #1: tid=3078226
0x40050f <+15>: movl $0x0, -0x8(%rbp)

>>> Before, some runs of the script up to this point lead to empty traces

(lldb) b 6
Breakpoint 3: where = a.out`main + 42 at main.cpp:6:14, address = 0x000000000040052a
(lldb) c
Process 3092991 resuming
Process 3092991 stopped
* thread #1, name = 'a.out', stop reason = breakpoint 3.1
frame #0: 0x000000000040052a a.out`main at main.cpp:6:14
(lldb) processor-trace show-instr-log thread #1: tid=3092991
0x40050f <+15>: movl $0x0, -0x8(%rbp)
0x400516 <+22>: movl $0x0, -0xc(%rbp)
0x40051d <+29>: cmpl $0x2710, -0xc(%rbp) ; imm = 0x2710
0x400524 <+36>: jge 0x400546 ; <+70> at main.cpp
0x400524 <+36>: jge 0x400546 ; <+70> at main.cpp

>>> The trace was re-enabled correctly and includes the instruction of the
first reading.
```
Those instructions correspond to these lines
```
3 int main() {
4 int z = 0;
5 for (int i = 0; i < 10000; i++) {
6 z += fun(z)
...
```

Differential Revision: https://reviews.llvm.org/D85241

Author: walter-erquinigo

lldb/source/Plugins/Process/Linux/ProcessorTrace.cpp

@@ -17,6 +17,7 @@
 #include "ProcessorTrace.h"
 #include "lldb/Host/linux/Support.h"
 
+#include <sys/ioctl.h>
 #include <sys/syscall.h>
 
 using namespace lldb;
@@ -273,6 +274,23 @@ ProcessorTraceMonitor::ReadPerfTraceAux(llvm::MutableArrayRef<uint8_t> &buffer,
 #ifndef PERF_ATTR_SIZE_VER5
   llvm_unreachable("perf event not supported");
 #else
+  // Disable the perf event to force a flush out of the CPU's internal buffer.
+  // Besides, we can guarantee that the CPU won't override any data as we are
+  // reading the buffer.
+  //
+  // The Intel documentation says:
+  //
+  // Packets are first buffered internally and then written out asynchronously.
+  // To collect packet output for postprocessing, a collector needs first to
+  // ensure that all packet data has been flushed from internal buffers.
+  // Software can ensure this by stopping packet generation by clearing
+  // IA32_RTIT_CTL.TraceEn (see “Disabling Packet Generation” in
+  // Section 35.2.7.2).
+  //
+  // This is achieved by the PERF_EVENT_IOC_DISABLE ioctl request, as mentioned
+  // in the man page of perf_event_open.
+  ioctl(*m_fd, PERF_EVENT_IOC_DISABLE);
+
   Log *log(ProcessPOSIXLog::GetLogIfAllCategoriesSet(POSIX_LOG_PTRACE));
   Status error;
   uint64_t head = m_mmap_meta->aux_head;
@@ -293,6 +311,9 @@ ProcessorTraceMonitor::ReadPerfTraceAux(llvm::MutableArrayRef<uint8_t> &buffer,
 
   ReadCyclicBuffer(buffer, GetAuxBuffer(), static_cast<size_t>(head), offset);
   LLDB_LOG(log, "ReadCyclic BUffer Done");
+
+  // Reenable tracing now we have read the buffer
+  ioctl(*m_fd, PERF_EVENT_IOC_ENABLE);
   return error;
 #endif
 }