From a51951c25813b8d6c763f51d78d3756d9ca85ee1 Mon Sep 17 00:00:00 2001 From: Kevin Buettner Date: Thu, 14 May 2020 11:10:14 -0700 Subject: [PATCH] Disable record btrace bts support for AMD processors Some Intel processors implement a Branch Trace Store (BTS) which GDB uses for reverse execution support via the "record btrace bts" command. I have been unable to find a description of a similar feature in a recent (April 2020) AMD64 architecture reference: https://www.amd.com/system/files/TechDocs/40332.pdf While it is the case that AMD processors have an LBR (last branch record) bit in the DebugCtl MSR, it seems that it affects only four MSRs when enabled. The names of these MSRs are LastBranchToIP, LastBranchFromIP, LastIntToIP, and LastIntFromIP. I can find no mention of anything more extensive. While looking at an Intel architecture document, I noticed that Intel's P6 family from the mid-90s had registers of the same name. Therefore... This commit disables "record btrace bts" support in GDB for AMD processors. Using the test case from gdb.base/break.exp, the sessions below show the expected behavior (run on a machine with an Intel processor) versus that on a machine with an AMD processor. The AMD processor in question is reported as follows by "lscpu": AMD Ryzen Threadripper 2950X 16-Core Processor . Finally, I'll note that the AMD machine is actually a VM, but I see similar behavior on both the virtualization host and the VM. Intel machine - Desired behavior: [kevinb@mohave gdb]$ ./gdb -q testsuite/outputs/gdb.base/break/break Reading symbols from testsuite/outputs/gdb.base/break/break... (gdb) start Temporary breakpoint 1 at 0x401179: file /home/kevinb/sourceware-git/native-build/bld/../../binutils-gdb/gdb/testsuite/gdb.base/break.c, line 43. Starting program: /home/kevinb/sourceware-git/native-build/bld/gdb/testsuite/outputs/gdb.base/break/break Temporary breakpoint 1, main (argc=1, argv=0x7fffffffd748, envp=0x7fffffffd758) at /home/kevinb/sourceware-git/native-build/bld/../../binutils-gdb/gdb/testsuite/gdb.base/break.c:43 43 if (argc == 12345) { /* an unlikely value < 2^16, in case uninited */ /* set breakpoint 6 here */ (gdb) record btrace (gdb) b factorial Breakpoint 2 at 0x40121b: file /home/kevinb/sourceware-git/native-build/bld/../../binutils-gdb/gdb/testsuite/gdb.base/break.c, line 63. (gdb) c Continuing. Breakpoint 2, factorial (value=6) at /home/kevinb/sourceware-git/native-build/bld/../../binutils-gdb/gdb/testsuite/gdb.base/break.c:63 63 if (value > 1) { /* set breakpoint 7 here */ (gdb) info record Active record target: record-btrace Recording format: Branch Trace Store. Buffer size: 64kB. Recorded 768 instructions in 22 functions (0 gaps) for thread 1 (process 19215). (gdb) record function-call-history 13 do_lookup_x 14 _dl_lookup_symbol_x 15 _dl_fixup 16 _dl_runtime_resolve_xsavec 17 atoi 18 strtoq 19 ____strtoll_l_internal 20 atoi 21 main 22 factorial (gdb) record instruction-history 759 0x00007ffff7ce0917 <____strtoll_l_internal+647>: pop %r15 760 0x00007ffff7ce0919 <____strtoll_l_internal+649>: retq 761 0x00007ffff7cdd064 : add $0x8,%rsp 762 0x00007ffff7cdd068 : retq 763 0x00000000004011b1 : mov %eax,%edi 764 0x00000000004011b3 : callq 0x401210 765 0x0000000000401210 : push %rbp 766 0x0000000000401211 : mov %rsp,%rbp 767 0x0000000000401214 : sub $0x10,%rsp 768 0x0000000000401218 : mov %edi,-0x4(%rbp) AMD machine - Wrong behavior: [kev@f32-1 gdb]$ ./gdb -q testsuite/outputs/gdb.base/break/break Reading symbols from testsuite/outputs/gdb.base/break/break... (gdb) start Temporary breakpoint 1 at 0x401179: file /ironwood1/sourceware-git/f32-master/bld/../../worktree-master/gdb/testsuite/gdb.base/break.c, line 43. Starting program: /mesquite2/sourceware-git/f32-master/bld/gdb/testsuite/outputs/gdb.base/break/break Temporary breakpoint 1, main (argc=1, argv=0x7fffffffd5b8, envp=0x7fffffffd5c8) at /ironwood1/sourceware-git/f32-master/bld/../../worktree-master/gdb/testsuite/gdb.base/break.c:43 43 if (argc == 12345) { /* an unlikely value < 2^16, in case uninited */ /* set breakpoint 6 here */ (gdb) record btrace (gdb) b factorial Breakpoint 2 at 0x40121b: file /ironwood1/sourceware-git/f32-master/bld/../../worktree-master/gdb/testsuite/gdb.base/break.c, line 63. (gdb) c Continuing. Breakpoint 2, factorial (value=6) at /ironwood1/sourceware-git/f32-master/bld/../../worktree-master/gdb/testsuite/gdb.base/break.c:63 63 if (value > 1) { /* set breakpoint 7 here */ (gdb) info record Active record target: record-btrace Recording format: Branch Trace Store. Buffer size: 64kB. warning: Recorded trace may be incomplete at instruction 7737 (pc = 0x405000). warning: Recorded trace may be incomplete at instruction 7739 (pc = 0x0). Recorded 7740 instructions in 46 functions (2 gaps) for thread 1 (process 1402911). (gdb) record function-call-history 37 ?? 38 values 39 some_enum_global 40 ?? 41 some_union_global 42 some_variable 43 ?? 44 [decode error (2): unknown instruction] 45 ?? 46 [decode error (2): unknown instruction] (gdb) record instruction-history 7730 0x0000000000404ff3: add %al,(%rax) 7731 0x0000000000404ff5: add %al,(%rax) 7732 0x0000000000404ff7: add %al,(%rax) 7733 0x0000000000404ff9: add %al,(%rax) 7734 0x0000000000404ffb: add %al,(%rax) 7735 0x0000000000404ffd: add %al,(%rax) 7736 0x0000000000404fff: .byte 0x0 7737 0x0000000000405000: Cannot access memory at address 0x405000 Lastly, I'll note that I see a lot of gdb.btrace failures without this commit. Worse still, the results aren't always the same which causes a lot of noise when comparing test results. gdbsupport/ChangeLog: * btrace-common.h (btrace_cpu_vendor): Add CV_AMD. gdb/ChangeLog: * nat/linux-btrace.c (btrace_this_cpu): Add check for AMD processors. (cpu_supports_bts): Add CV_AMD case. --- gdb/ChangeLog | 6 ++++++ gdb/nat/linux-btrace.c | 6 ++++++ gdbsupport/ChangeLog | 4 ++++ gdbsupport/btrace-common.h | 5 ++++- 4 files changed, 20 insertions(+), 1 deletion(-) diff --git a/gdb/ChangeLog b/gdb/ChangeLog index 42118e731f5..44ccc180d2a 100644 --- a/gdb/ChangeLog +++ b/gdb/ChangeLog @@ -1,3 +1,9 @@ +2020-05-14 Kevin Buettner + + * nat/linux-btrace.c (btrace_this_cpu): Add check for AMD + processors. + (cpu_supports_bts): Add CV_AMD case. + 2020-05-14 Laurent Morichetti Simon Marchi diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c index e972a072279..b87faf98b2c 100644 --- a/gdb/nat/linux-btrace.c +++ b/gdb/nat/linux-btrace.c @@ -90,6 +90,9 @@ btrace_this_cpu (void) cpu.model += (cpuid >> 12) & 0xf0; } } + else if (ebx == signature_AMD_ebx && ecx == signature_AMD_ecx + && edx == signature_AMD_edx) + cpu.vendor = CV_AMD; } return cpu; @@ -406,6 +409,9 @@ cpu_supports_bts (void) case CV_INTEL: return intel_supports_bts (&cpu); + + case CV_AMD: + return 0; } } diff --git a/gdbsupport/ChangeLog b/gdbsupport/ChangeLog index 636a3d34e14..e7374abfc26 100644 --- a/gdbsupport/ChangeLog +++ b/gdbsupport/ChangeLog @@ -1,3 +1,7 @@ +2020-05-14 Kevin Buettner + + * btrace-common.h (btrace_cpu_vendor): Add CV_AMD. + 2020-05-14 Tankut Baris Aktemur * common-regcache.h (regcache_read_pc_protected): New function diff --git a/gdbsupport/btrace-common.h b/gdbsupport/btrace-common.h index c9dc683044f..226c67b297d 100644 --- a/gdbsupport/btrace-common.h +++ b/gdbsupport/btrace-common.h @@ -74,7 +74,10 @@ enum btrace_cpu_vendor CV_UNKNOWN, /* Intel. */ - CV_INTEL + CV_INTEL, + + /* AMD. */ + CV_AMD }; /* A cpu identifier. */ -- 2.30.2