[PATCH v2] Disable record btrace bts support for AMD processors

[Kevin Buettner <kevinb@redhat.com>]

[Note: The actual patch for v2 is unchanged.  I've fixed a mistake
in the commit log and expanded it a bit too.]

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 <atoi+20>:	add    $0x8,%rsp
762	   0x00007ffff7cdd068 <atoi+24>:	retq
763	   0x00000000004011b1 <main+75>:	mov    %eax,%edi
764	   0x00000000004011b3 <main+77>:	callq  0x401210 <factorial>
765	   0x0000000000401210 <factorial+0>:	push   %rbp
766	   0x0000000000401211 <factorial+1>:	mov    %rsp,%rbp
767	   0x0000000000401214 <factorial+4>:	sub    $0x10,%rsp
768	   0x0000000000401218 <factorial+8>:	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/nat/linux-btrace.c     | 6 ++++++
 gdbsupport/btrace-common.h | 5 ++++-
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/gdb/nat/linux-btrace.c b/gdb/nat/linux-btrace.c
index e972a07227..b87faf98b2 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/btrace-common.h b/gdbsupport/btrace-common.h
index c9dc683044..226c67b297 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.  */
-- 
2.26.2