- qemu CVS, without using the kqemu module
- Solaris x86 guest
- I'm trying to debug a user program inside the Solaris x86 guest:
$ mdb /bin/date
> main:b
> :r
(note: mdb uses a breakpoint inside the target's dynamic linker
ld.so.1 and single steps over that breakpoint during target
program startup)
When kqemu isn't available, single stepping a user programm in the
/bin/mdb debugger in a Solaris x86 guest doesn't work.
The Solaris x86 kernel verifies that the "BS (single step) flag (bit
14)" in the DR6 debug status register is set when a user
program gets an exception #1 (EXCP01_SSTP).
qemu currently doesn't set this bit for exception #1 (EXCP01_SSTP).
The Solaris x86 kernel complains with the message:
Unexpected INT 1 in user mode, dr6=0
At this point the Solaris x86 guest kernel is stuck in an endless
loop with "Unexpected INT 1 in user mode, dr6=0" messages.
Workaround:
===========
For the x86 platform only: use the kqemu module.
Suggested fix:
==============
Set the 0x4000 bit in DR6 when single stepping.
See the attached patch. With this patch applied, debugging a user
program works.
diff -ru /tmp/qemu-cvs/target-i386/exec.h qemu-cvs/target-i386/exec.h
--- /tmp/qemu-cvs/target-i386/exec.h 2006-09-25 09:52:23.000000000 +0200
+++ qemu-cvs/target-i386/exec.h 2007-03-10 21:20:22.804313251 +0100
@@ -191,6 +191,7 @@
void helper_idivq_EAX_T0(void);
void helper_bswapq_T0(void);
void helper_cmpxchg8b(void);
+void helper_single_step(void);
void helper_cpuid(void);
void helper_enter_level(int level, int data32);
void helper_enter64_level(int level, int data64);
diff -ru /tmp/qemu-cvs/target-i386/helper.c qemu-cvs/target-i386/helper.c
--- /tmp/qemu-cvs/target-i386/helper.c 2007-02-09 22:10:08.000000000 +0100
+++ qemu-cvs/target-i386/helper.c 2007-03-10 21:13:09.708272230 +0100
@@ -1591,6 +1591,12 @@
CC_SRC = eflags;
}
+void helper_single_step()
+{
+ env->dr[6] |= 0x4000;
+ raise_exception(EXCP01_SSTP);
+}
+
void helper_cpuid(void)
{
uint32_t index;
diff -ru /tmp/qemu-cvs/target-i386/op.c qemu-cvs/target-i386/op.c
--- /tmp/qemu-cvs/target-i386/op.c 2007-02-09 22:10:08.000000000 +0100
+++ qemu-cvs/target-i386/op.c 2007-03-10 21:20:53.276293877 +0100
@@ -730,6 +730,11 @@
helper_cmpxchg8b();
}
+void OPPROTO op_single_step(void)
+{
+ helper_single_step();
+}
+
void OPPROTO op_movl_T0_0(void)
{
T0 = 0;
diff -ru /tmp/qemu-cvs/target-i386/translate.c qemu-cvs/target-i386/translate.c
--- /tmp/qemu-cvs/target-i386/translate.c 2007-01-24 10:07:52.000000000
+0100
+++ qemu-cvs/target-i386/translate.c 2007-03-10 21:49:06.287293924 +0100
@@ -2277,7 +2277,7 @@
if (s->singlestep_enabled) {
gen_op_debug();
} else if (s->tf) {
- gen_op_raise_exception(EXCP01_SSTP);
+ gen_op_single_step();
} else {
gen_op_movl_T0_0();
gen_op_exit_tb();
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