[Warning in advance]

This patch reintroduces the early KGDB_CONSOLE registration issue: If

you switch this on, trigger the debugger before any tty-providing

console was registered and you do not provide an explicit console=

parameter, you end up with a panic due to lacking /dev/console. But this

time I have a better fix (for register_console), will be posted

separately later on.
<--snip-->
In case "kgdbwait" is passed as kernel parameter, KGDB tries to set up

and connect to the front-end already during early_param evaluation. This

fails on x86 as the exception stack is not yet initialized, effectively

delaying kgdbwait until late-init.
Therefore, this patch hooks into the x86 trap initialization and

re-triggers the KGDB setup, including a potential early rendezvous with

gdb. As a precondition, KGDB's setup states are refactored once again to

allow multiple invocations of kgdb_early_entry and correct tracking of

pending kgdbwait requests.
Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
---

 arch/x86/kernel/traps_32.c |    4 +++

 arch/x86/kernel/traps_64.c |    4 +++

 include/linux/kgdb.h       |    7 ++++-

 kernel/kgdb.c              |   55 ++++++++++++++++++++++++---------------------

 4 files changed, 44 insertions(+), 26 deletions(-)
Index: b/arch/x86/kernel/traps_32.c

===================================================================

--- a/arch/x86/kernel/traps_32.c

+++ b/arch/x86/kernel/traps_32.c

@@ -25,6 +25,7 @@

 #include <linux/utsname.h>

 #include <linux/kprobes.h>

 #include <linux/kexec.h>

+#include <linux/kgdb.h>

 #include <linux/unwind.h>

 #include <linux/uaccess.h>

 #include <linux/nmi.h>

@@ -1215,6 +1216,9 @@ void __init trap_init(void)

 	 */

 	cpu_init();
+	/* With the TSS set up, it's now save to arm early KGDB. */

+	kgdb_early_entry();

+

 	trap_init_hook();

 }
Index: b/arch/x86/kernel/traps_64.c

===================================================================

--- a/arch/x86/kern...
[ message continues ]

[Here comes a rebased version against latest x86/mm]
In case "kgdbwait" is passed as kernel parameter, KGDB tries to set up

and connect to the front-end already during early_param evaluation. This

fails on x86 as the exception stack is not yet initialized, effectively

delaying kgdbwait until late-init.
Therefore, this patch hooks into the x86 trap initialization and

re-triggers the KGDB setup, including a potential early rendezvous with

gdb. As a precondition, KGDB's setup states are refactored once again to

allow multiple invocations of kgdb_early_entry and correct tracking of

pending kgdbwait requests.
Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
---

 arch/x86/kernel/traps_32.c |    4 +++

 arch/x86/kernel/traps_64.c |    4 +++

 include/linux/kgdb.h       |    7 +++++-

 kernel/kgdb.c              |   52 +++++++++++++++++++++++++--------------------

 4 files changed, 44 insertions(+), 23 deletions(-)
Index: b/arch/x86/kernel/traps_32.c

===================================================================

--- a/arch/x86/kernel/traps_32.c

+++ b/arch/x86/kernel/traps_32.c

@@ -25,6 +25,7 @@

 #include <linux/utsname.h>

 #include <linux/kprobes.h>

 #include <linux/kexec.h>

+#include <linux/kgdb.h>

 #include <linux/unwind.h>

 #include <linux/uaccess.h>

 #include <linux/nmi.h>

@@ -1215,6 +1216,9 @@ void __init trap_init(void)

 	 */

 	cpu_init();
+	/* With the TSS set up, it's now save to arm early KGDB. */

+	kgdb_early_entry();

+

 	trap_init_hook();

 }
Index: b/arch/x86/kernel/traps_64.c

===================================================================

--- a/arch/x86/kernel/traps_64.c

+++ b/arch/x86/kernel/traps_64.c

@@ -27,6 +27,7 @@

 #include <linux/nmi.h>

 #include <linux/kprobes.h>

 #include <linux/kexec.h>

+#include <linux/kgdb.h>

 #include <linux/unwind.h>

 #include <linux/uaccess.h>

 #include <linux/bug.h>

@@ -1162,6 +1163,9 @@ void __init trap_init(void)

 	 *...
[ message continues ]

On 01/31/2008 01:36 AM,  Jan Kiszka was caught saying:

 > Jan Kiszka wrote:

 >> George Anzinger wrote:

 >>> On 01/30/2008 04:08 PM,  Jan Kiszka was caught saying:

 >>>> [Here comes a rebased version against latest x86/mm]

 >>>>

 >>>> In case "kgdbwait" is passed as kernel parameter, KGDB tries to set up

 >>>> and connect to the front-end already during early_param evaluation.

 >>>> This

 >>>> fails on x86 as the exception stack is not yet initialized,

effectively

 >>>> delaying kgdbwait until late-init.

 >>>

 >>> I wonder how much work it would take to just set up the exception

 >>> stack and proceed.  After all the kgbdwait is there to help debug

 >>> very early kernel code...

 >>

 >> In principle a valid question, but I'm not the one to answer it. I

 >> would not feel very well if I had to reorder this critical setup code.

 >> Look, we would have to move trap_init in start_kernel before

 >> parse_early_param, and that would affect _every_ arch...
I can not speak to other archs, but for x86 I called trap_init from the

code that caught the kgdbwait.  At that time (since I retired, I have

not looked at the actual kernel code) it could be called again later by

the kernel code.  I.e. I did not try to reorder the kernel bring up

code, but just added an additional call to trap_init and then only in

the case of finding a kgdbwait.
As such, this would need to be arch specific...
 >>

 >

 > BTW, do you know if EXCEPTION_STACK_READY fails for other archs in

 > parse_early_param as well? It should, because my under standing of

 > trap_init is that it's the functions to arm things like... exception

 > handlers? And that raises the question of the deeper purpose of this

 > check (and the invocation of kgdb_early_init from the argument parsing

 > function). Sigh, KGDB is still a quite impr...
[ message continues ]

Meanwhile I realized that there is early_trap_init - for x86-32 only! I

assume now we are only lacking the same for x86-64 to get kgdb running

there already during early_param-parsing.
Jan

--

Looks like that was the key. Thanks for pointing me at this, George.

Here the updated patch:
------snip-------
This cleans up the early entry of kgdb. It introduces early_trap_init

for x86-64, reloads the idt register also in the 32-bit variant, removes

the now unneeded EXCEPTION_STACK_READY construction, and matures the

init-state machine of kgdb.
Signed-off-by: Jan Kiszka <jan.kiszka@web.de>
---

 arch/x86/kernel/setup_64.c |    4 +++

 arch/x86/kernel/traps_32.c |    3 +-

 arch/x86/kernel/traps_64.c |   10 ++++++++-

 include/asm-x86/kgdb.h     |    3 --

 include/linux/kgdb.h       |    7 +-----

 kernel/kgdb.c              |   47 +++++++++++++++++++--------------------------

 6 files changed, 37 insertions(+), 37 deletions(-)
Index: b/arch/x86/kernel/traps_32.c

===================================================================

--- a/arch/x86/kernel/traps_32.c

+++ b/arch/x86/kernel/traps_32.c

@@ -1137,12 +1137,13 @@ asmlinkage void math_emulate(long arg)
 #endif /* CONFIG_MATH_EMULATION */
-/* Some traps need to be set early. */

+/* Set of traps needed for early debugging. */

 void __init early_trap_init(void)

 {

 	set_intr_gate(1, &debug);

 	set_system_intr_gate(3, &int3); /* int3 can be called from all */

 	set_intr_gate(14, &page_fault);

+	load_idt(&idt_descr);

 }
 void __init trap_init(void)

Index: b/arch/x86/kernel/traps_64.c

===================================================================

--- a/arch/x86/kernel/traps_64.c

+++ b/arch/x86/kernel/traps_64.c

@@ -1129,6 +1129,15 @@ asmlinkage void math_state_restore(void)

 }

 EXPORT_SYMBOL_GPL(math_state_restore);
+/* Set of traps needed for early debugging. */

+void __init early_trap_init(void)

+{

+	set_intr_gate(1, &debug);

+	set_intr_gate(3, &int3);

+	set_intr_gate(14, &page_fault);

+	load_idt((const struct desc_ptr *)&idt_descr);

+}

+

 void __init trap_init(void)

 {

 	set_intr_gate(0,&divide_error);

@@ -1145,7 +1154,6 @@ void __init trap_ini...
[ message continues ]