Patchwork [07/11] ARM: Add KGDB/KDB FIQ debugger generic code

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Submitter Anton Vorontsov
Date July 30, 2012, 11:58 a.m.
Message ID <1343649500-18491-7-git-send-email-anton.vorontsov@linaro.org>
Download mbox | patch
Permalink /patch/174007/
State New
Headers show

Comments

Anton Vorontsov - July 30, 2012, 11:58 a.m.
The FIQ debugger may be used to debug situations when the kernel stuck
in uninterruptable sections, e.g. the kernel infinitely loops or
deadlocked in an interrupt or with interrupts disabled.

By default KGDB FIQ is disabled in runtime, but can be enabled with
kgdb_fiq.enable=1 kernel command line option.

Signed-off-by: Anton Vorontsov <anton.vorontsov@linaro.org>
---
 arch/arm/Kconfig                 |   18 +++++++
 arch/arm/include/asm/kgdb.h      |    8 +++
 arch/arm/kernel/Makefile         |    1 +
 arch/arm/kernel/kgdb_fiq.c       |  101 ++++++++++++++++++++++++++++++++++++++
 arch/arm/kernel/kgdb_fiq_entry.S |   79 +++++++++++++++++++++++++++++
 5 files changed, 207 insertions(+)
 create mode 100644 arch/arm/kernel/kgdb_fiq.c
 create mode 100644 arch/arm/kernel/kgdb_fiq_entry.S
Russell King - ARM Linux - July 30, 2012, 2:07 p.m.
On Mon, Jul 30, 2012 at 04:58:16AM -0700, Anton Vorontsov wrote:
> +	.align	5
> +__fiq_svc:
> +	svc_entry
> +	fiq_handler
> +	svc_exit r5				@ return from exception
> + UNWIND(.fnend		)
> +ENDPROC(__fiq_svc)
> +	.ltorg
> +
> +	.align	5
> +__fiq_usr:
> +	usr_entry
> +	kuser_cmpxchg_check
> +	fiq_handler
> +	get_thread_info tsk
> +	mov	why, #0
> +	b	ret_to_user_from_irq
> + UNWIND(.fnend		)
> +ENDPROC(__fiq_usr)
> +	.ltorg
> +
> +	.global kgdb_fiq_handler
> +kgdb_fiq_handler:
> +
> +	vector_stub	fiq, FIQ_MODE, 4
> +
> +	.long	__fiq_usr			@  0  (USR_26 / USR_32)
> +	.long	__fiq_svc			@  1  (FIQ_26 / FIQ_32)
> +	.long	__fiq_svc			@  2  (IRQ_26 / IRQ_32)
> +	.long	__fiq_svc			@  3  (SVC_26 / SVC_32)
> +	.long	__fiq_svc			@  4
> +	.long	__fiq_svc			@  5
> +	.long	__fiq_svc			@  6
> +	.long	__fiq_svc			@  7
> +	.long	__fiq_svc			@  8
> +	.long	__fiq_svc			@  9
> +	.long	__fiq_svc			@  a
> +	.long	__fiq_svc			@  b
> +	.long	__fiq_svc			@  c
> +	.long	__fiq_svc			@  d
> +	.long	__fiq_svc			@  e
> +	.long	__fiq_svc			@  f

I am not convinced that this does not cause loss of state from the parent
context.  Let's review what happens when a FIQ is received from SVC mode
with the above code.

- The CPU will be in SVC mode.
- FIQ received.
- CPU saves CPSR into SPSR_fiq and PC into LR_fiq, and jumps to the FIQ
  vector.
- We apply the 4 byte correction to LR_fiq, and store r0, LR_fiq and
  SPSR_fiq to the FIQ 'stack'
- We switch to SVC mode and jump to __fiq_svc
- svc_entry:
  - adjusts the SVC stack pointer down, and saves r1 - r12
  - loads r0, LR_fiq and SPSR_fiq and saves them as ARM_r0, ARM_pc, ARM_cpsr
    into the pt_regs
  - the original value of the SVC stack pointer is saved as ARM_r13
  - LR_svc is saved as ARM_r14

At this point, we have saved everything *except* for the SPSR_svc register.

Now, when we return from the above, we use svc_exit:
- write SPSR_svc with ARM_cpsr (from SPSR_fiq)
- load r0-pc from the pt_regs and load CPSR from SPSR_svc

Now the thing here is that even if we did preserve SPSR_svc, with the
above exit sequence, there is _no_ way to preserve the value of SPSR_svc.
Normally, this doesn't matter because we know that the regions we care
about this have IRQs disabled.

However, what this means, if we receive an FIQ and use this path from any
part of the kernel which expects SPSR_svc to be preserved (eg, the exit
path from any exception) the kernel will blow up.

I guess you could do something like this instead:
- disable FIQs
- load SPSR_svc with a saved value of it from entry.
- load r1-r14 from ARM_r1..ARM_lr
- switch to FIQ mode
- load SPSR_fiq from saved ARM_cpsr
- load r0 from ARM_r0
- load pc from ARM_pc

So, maybe something like this for the svc return path:

	cpsid	f
	ldr	r1, [saved_spsr_svc]
	mov	r0, sp
	mrs	spsr_cxsf, r1
	ldmib	r0, {r1 - r14}
	msr	cpsr_c, #FIQ_MODE | PSR_I_BIT | PSR_F_BIT
	add	r7, r0, #S_PC
	ldr	r8, [r0, #S_CPSR]
	mrs	spsr_cxsf, r8
	ldr	r0, [r0, #S_R0]
	ldmia	r7, {pc}^

Patch

diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
index a91009c..ae6ddf1 100644
--- a/arch/arm/Kconfig
+++ b/arch/arm/Kconfig
@@ -172,6 +172,24 @@  config GENERIC_ISA_DMA
 config FIQ
 	bool
 
+config ARCH_MIGHT_HAVE_FIQ
+	bool
+
+config KGDB_FIQ
+	bool "KGDB/KDB FIQ debugger"
+	depends on KGDB_KDB && ARCH_MIGHT_HAVE_FIQ && !THUMB2_KERNEL
+	select FIQ
+	help
+	  The FIQ debugger may be used to debug situations when the
+	  kernel stuck in uninterruptable sections, e.g. the kernel
+	  infinitely loops or deadlocked in an interrupt or with
+	  interrupts disabled.
+
+	  By default KGDB FIQ is disabled in runtime, but can be
+	  enabled with kgdb_fiq.enable=1 kernel command line option.
+
+	  If unsure, say N.
+
 config NEED_RET_TO_USER
 	bool
 
diff --git a/arch/arm/include/asm/kgdb.h b/arch/arm/include/asm/kgdb.h
index 48066ce..807e547 100644
--- a/arch/arm/include/asm/kgdb.h
+++ b/arch/arm/include/asm/kgdb.h
@@ -11,6 +11,8 @@ 
 #define __ARM_KGDB_H__
 
 #include <linux/ptrace.h>
+#include <linux/linkage.h>
+#include <asm/exception.h>
 
 /*
  * GDB assumes that we're a user process being debugged, so
@@ -47,6 +49,12 @@  static inline void arch_kgdb_breakpoint(void)
 extern void kgdb_handle_bus_error(void);
 extern int kgdb_fault_expected;
 
+extern char kgdb_fiq_handler;
+extern char kgdb_fiq_handler_end;
+asmlinkage void __exception_irq_entry kgdb_fiq_do_handle(struct pt_regs *regs);
+extern int __init kgdb_register_fiq(unsigned int mach_kgdb_fiq,
+		     void (*mach_kgdb_enable_fiq)(unsigned int irq, bool on),
+		     bool (*mach_is_kgdb_fiq)(unsigned int irq));
 #endif /* !__ASSEMBLY__ */
 
 /*
diff --git a/arch/arm/kernel/Makefile b/arch/arm/kernel/Makefile
index 7ad2d5c..5aa079b 100644
--- a/arch/arm/kernel/Makefile
+++ b/arch/arm/kernel/Makefile
@@ -56,6 +56,7 @@  obj-$(CONFIG_ATAGS_PROC)	+= atags.o
 obj-$(CONFIG_OABI_COMPAT)	+= sys_oabi-compat.o
 obj-$(CONFIG_ARM_THUMBEE)	+= thumbee.o
 obj-$(CONFIG_KGDB)		+= kgdb.o
+obj-$(CONFIG_KGDB_FIQ)		+= kgdb_fiq_entry.o kgdb_fiq.o
 obj-$(CONFIG_ARM_UNWIND)	+= unwind.o
 obj-$(CONFIG_HAVE_TCM)		+= tcm.o
 obj-$(CONFIG_OF)		+= devtree.o
diff --git a/arch/arm/kernel/kgdb_fiq.c b/arch/arm/kernel/kgdb_fiq.c
new file mode 100644
index 0000000..72a62c7
--- /dev/null
+++ b/arch/arm/kernel/kgdb_fiq.c
@@ -0,0 +1,101 @@ 
+/*
+ * KGDB FIQ entry
+ *
+ * Copyright 2010 Google, Inc.
+ *		  Arve Hjønnevåg <arve@android.com>
+ *		  Colin Cross <ccross@android.com>
+ * Copyright 2012 Linaro Ltd.
+ *		  Anton Vorontsov <anton.vorontsov@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published
+ * by the Free Software Foundation.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <linux/init.h>
+#include <linux/slab.h>
+#include <linux/errno.h>
+#include <linux/hardirq.h>
+#include <linux/kgdb.h>
+#include <asm/fiq.h>
+#include <asm/exception.h>
+
+static int kgdb_fiq_enabled;
+module_param_named(enable, kgdb_fiq_enabled, int, 0600);
+MODULE_PARM_DESC(enable, "set to 1 to enable FIQ KGDB");
+
+static unsigned int kgdb_fiq;
+static void (*kgdb_enable_fiq)(unsigned int irq, bool on);
+static bool (*is_kgdb_fiq)(unsigned int irq);
+
+asmlinkage void __exception_irq_entry kgdb_fiq_do_handle(struct pt_regs *regs)
+{
+	if (!is_kgdb_fiq(kgdb_fiq))
+		return;
+	dbg_io_ops->clear_irqs();
+
+	nmi_enter();
+	kgdb_handle_exception(1, 0, 0, regs);
+	nmi_exit();
+}
+
+static struct fiq_handler kgdb_fiq_desc = {
+	.name = "kgdb",
+};
+
+static long kgdb_fiq_setup_stack(void *info)
+{
+	struct pt_regs regs;
+
+	regs.ARM_sp = __get_free_pages(GFP_KERNEL, THREAD_SIZE_ORDER) +
+			THREAD_START_SP;
+	WARN_ON(!regs.ARM_sp);
+
+	set_fiq_regs(&regs);
+	return 0;
+}
+
+int kgdb_arch_enable_nmi(bool on)
+{
+	static int cnt;
+
+	if (cnt > 0 && on)
+		return cnt;
+	cnt += on ? 1 : -1;
+	kgdb_enable_fiq(kgdb_fiq, cnt > 0);
+	return cnt;
+}
+
+int __init kgdb_register_fiq(unsigned int mach_kgdb_fiq,
+		void (*mach_kgdb_enable_fiq)(unsigned int irq, bool on),
+		bool (*mach_is_kgdb_fiq)(unsigned int irq))
+{
+	int err;
+	int cpu;
+
+	if (!kgdb_fiq_enabled)
+		return -ENODEV;
+	if (kgdb_fiq)
+		return -EBUSY;
+
+	kgdb_fiq = mach_kgdb_fiq;
+	kgdb_enable_fiq = mach_kgdb_enable_fiq;
+	is_kgdb_fiq = mach_is_kgdb_fiq;
+
+	err = claim_fiq(&kgdb_fiq_desc);
+	if (err) {
+		pr_warn("%s: unable to claim fiq", __func__);
+		return err;
+	}
+
+	for_each_possible_cpu(cpu)
+		work_on_cpu(cpu, kgdb_fiq_setup_stack, NULL);
+
+	set_fiq_handler(&kgdb_fiq_handler,
+			&kgdb_fiq_handler_end - &kgdb_fiq_handler);
+
+	kgdb_arch_enable_nmi(1);
+	return 0;
+}
diff --git a/arch/arm/kernel/kgdb_fiq_entry.S b/arch/arm/kernel/kgdb_fiq_entry.S
new file mode 100644
index 0000000..7be3726
--- /dev/null
+++ b/arch/arm/kernel/kgdb_fiq_entry.S
@@ -0,0 +1,79 @@ 
+/*
+ * KGDB FIQ entry
+ *
+ * Copyright 1996,1997,1998 Russell King.
+ * Copyright 2012 Linaro Ltd.
+ *		  Anton Vorontsov <anton.vorontsov@linaro.org>
+ *
+ * This program is free software; you can redistribute it and/or modify it
+ * under the terms of the GNU General Public License version 2 as published
+ * by the Free Software Foundation.
+ */
+
+#include <linux/linkage.h>
+#include <asm/assembler.h>
+#include <asm/memory.h>
+#include <asm/unwind.h>
+#include "entry-header.S"
+
+	.text
+
+@ This is needed for usr_entry/alignment_trap
+.LCcralign:
+	.long	cr_alignment
+.LCdohandle:
+	.long	kgdb_fiq_do_handle
+
+	.macro	fiq_handler
+	ldr	r1, =.LCdohandle
+	mov	r0, sp
+	adr	lr, BSYM(9997f)
+	ldr	pc, [r1]
+9997:
+	.endm
+
+	.align	5
+__fiq_svc:
+	svc_entry
+	fiq_handler
+	svc_exit r5				@ return from exception
+ UNWIND(.fnend		)
+ENDPROC(__fiq_svc)
+	.ltorg
+
+	.align	5
+__fiq_usr:
+	usr_entry
+	kuser_cmpxchg_check
+	fiq_handler
+	get_thread_info tsk
+	mov	why, #0
+	b	ret_to_user_from_irq
+ UNWIND(.fnend		)
+ENDPROC(__fiq_usr)
+	.ltorg
+
+	.global kgdb_fiq_handler
+kgdb_fiq_handler:
+
+	vector_stub	fiq, FIQ_MODE, 4
+
+	.long	__fiq_usr			@  0  (USR_26 / USR_32)
+	.long	__fiq_svc			@  1  (FIQ_26 / FIQ_32)
+	.long	__fiq_svc			@  2  (IRQ_26 / IRQ_32)
+	.long	__fiq_svc			@  3  (SVC_26 / SVC_32)
+	.long	__fiq_svc			@  4
+	.long	__fiq_svc			@  5
+	.long	__fiq_svc			@  6
+	.long	__fiq_svc			@  7
+	.long	__fiq_svc			@  8
+	.long	__fiq_svc			@  9
+	.long	__fiq_svc			@  a
+	.long	__fiq_svc			@  b
+	.long	__fiq_svc			@  c
+	.long	__fiq_svc			@  d
+	.long	__fiq_svc			@  e
+	.long	__fiq_svc			@  f
+
+	.global kgdb_fiq_handler_end
+kgdb_fiq_handler_end: