Author: Walter Erquinigo Date: 2020-10-23T16:36:42-07:00 New Revision: 48d8af98251613261805891a3476e9fb01932f43
URL: https://github.com/llvm/llvm-project/commit/48d8af98251613261805891a3476e9fb01932f43 DIFF: https://github.com/llvm/llvm-project/commit/48d8af98251613261805891a3476e9fb01932f43.diff LOG: [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 Added: Modified: lldb/source/Plugins/Process/Linux/ProcessorTrace.cpp Removed: ################################################################################ diff --git a/lldb/source/Plugins/Process/Linux/ProcessorTrace.cpp b/lldb/source/Plugins/Process/Linux/ProcessorTrace.cpp index 4817ef2ac3e0..9bb01f287a01 100644 --- a/lldb/source/Plugins/Process/Linux/ProcessorTrace.cpp +++ b/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 } _______________________________________________ lldb-commits mailing list lldb-commits@lists.llvm.org https://lists.llvm.org/cgi-bin/mailman/listinfo/lldb-commits