[all-commits] [llvm/llvm-project] 48d8af: [intel-pt] Disable/Enable tracing to guarantee the...

[walter erquinigo via All-commits]

  Branch: refs/heads/master
  Home:   https://github.com/llvm/llvm-project
  Commit: 48d8af98251613261805891a3476e9fb01932f43
      https://github.com/llvm/llvm-project/commit/48d8af98251613261805891a3476e9fb01932f43
  Author: Walter Erquinigo <a20012251 at gmail.com>
  Date:   2020-10-23 (Fri, 23 Oct 2020)

  Changed paths:
    M lldb/source/Plugins/Process/Linux/ProcessorTrace.cpp

  Log Message:
  -----------
  [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