x86: early boot debugging via FireWire (ohci1394_dma=early)

This patch adds a new configuration option, which adds support for a new
early_param which gets checked in arch/x86/kernel/setup_{32,64}.c:setup_arch()
to decide wether OHCI-1394 FireWire controllers should be initialized and
enabled for physical DMA access to allow remote debugging of early problems
like issues ACPI or other subsystems which are executed very early.

If the config option is not enabled, no code is changed, and if the boot
paramenter is not given, no new code is executed, and independent of that,
all new code is freed after boot, so the config option can be even enabled
in standard, non-debug kernels.

With specialized tools, it is then possible to get debugging information
from machines which have no serial ports (notebooks) such as the printk
buffer contents, or any data which can be referenced from global pointers,
if it is stored below the 4GB limit and even memory dumps of of the physical
RAM region below the 4GB limit can be taken without any cooperation from the
CPU of the host, so the machine can be crashed early, it does not matter.

In the extreme, even kernel debuggers can be accessed in this way. I wrote
a small kgdb module and an accompanying gdb stub for FireWire which allows
to gdb to talk to kgdb using remote remory reads and writes over FireWire.

An version of the gdb stub fore FireWire is able to read all global data
from a system which is running a a normal kernel without any kernel debugger,
without any interruption or support of the system's CPU. That way, e.g. the
task struct and so on can be read and even manipulated when the physical DMA
access is granted.

A HOWTO is included in this patch, in Documentation/debugging-via-ohci1394.txt
and I've put a copy online at
ftp://ftp.suse.de/private/bk/firewire/docs/debugging-via-ohci1394.txt

It also has links to all the tools which are available to make use of it
another copy of it is online at:
ftp://ftp.suse.de/private/bk/firewire/kernel/ohci1394_dma_early-v2.diff

Signed-Off-By: Bernhard Kaindl <bk@suse.de>
Tested-By: Thomas Renninger <trenn@suse.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

2 files changed, 22 insertions, 0 deletions

diff --git a/arch/x86/kernel/setup_32.c b/arch/x86/kernel/setup_32.c
index 9c0ef4945a58..62adc5f20be5 100644
--- a/arch/x86/kernel/setup_32.c
+++ b/arch/x86/kernel/setup_32.c
@@ -45,6 +45,7 @@
 #include <linux/dmi.h>
 #include <linux/pfn.h>
 #include <linux/pci.h>
+#include <linux/init_ohci1394_dma.h>
 
 #include <video/edid.h>
 
@@ -787,6 +788,16 @@ void __init setup_arch(char **cmdline_p)
         smp_alloc_memory(); /* AP processor realmode stacks in low memory*/
 #endif
         paging_init();
+
+        /*
+         * NOTE: On x86-32, only from this point on, fixmaps are ready for use.
+         */
+
+#ifdef CONFIG_PROVIDE_OHCI1394_DMA_INIT
+        if (init_ohci1394_dma_early)
+                init_ohci1394_dma_on_all_controllers();
+#endif
+
         remapped_pgdat_init();
         sparse_init();
         zone_sizes_init();
diff --git a/arch/x86/kernel/setup_64.c b/arch/x86/kernel/setup_64.c
index 697533e86822..77fb87bf6e5a 100644
--- a/arch/x86/kernel/setup_64.c
+++ b/arch/x86/kernel/setup_64.c
@@ -41,6 +41,7 @@
 #include <linux/dma-mapping.h>
 #include <linux/ctype.h>
 #include <linux/uaccess.h>
+#include <linux/init_ohci1394_dma.h>
 
 #include <asm/mtrr.h>
 #include <asm/uaccess.h>
@@ -253,6 +254,11 @@ void __attribute__((weak)) __init memory_setup(void)
        machine_specific_memory_setup();
 }
 
+/*
+ * setup_arch - architecture-specific boot-time initializations
+ *
+ * Note: On x86_64, fixmaps are ready for use even before this is called.
+ */
 void __init setup_arch(char **cmdline_p)
 {
         unsigned i;
@@ -302,6 +308,11 @@ void __init setup_arch(char **cmdline_p)
 
         parse_early_param();
 
+#ifdef CONFIG_PROVIDE_OHCI1394_DMA_INIT
+        if (init_ohci1394_dma_early)
+                init_ohci1394_dma_on_all_controllers();
+#endif
+
         finish_e820_parsing();
 
         early_gart_iommu_check();