arch: riscv: optionally stores a pointer to csf in struct arch_esf

The callee-saved-registers can be helpful to debug the state of
a core upon an exception, however, currently there's no way to
access that information in user-implemented
`k_sys_fatal_error_handler()`, even though the csf is already stored
in the stack.

This patch conditionally add a `csf` member in the `arch_esf` when
`CONFIG_EXTRA_EXCEPTION_INFO=y`*, which the `_isr_wrapper` would update
when a fatal error occurs before invoking `z_riscv_fatal_error_csf()`.

Functions such as `k_sys_fatal_error_handler()` would then be able
to access the callee-saved-registers at the time of exception via
`esf->csf`.

* For SoCs that select `RISCV_SOC_HAS_ISR_STACKING`, the
  `SOC_ISR_STACKING_ESF_DECLARE` has to include the `csf` member,
  otherwise the build would fail.

Signed-off-by: Yong Cong Sin <ycsin@meta.com>
Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>

arch/riscv/Kconfig

@@ -238,6 +238,14 @@ config RISCV_HART_MASK
 	  i.e. 128, 129, ..(0x80, 8x81, ..), this can be configured to 63 (0x7f)
 	  such that we can extract the bits that start from 0.
 
+config EXTRA_EXCEPTION_INFO
+	bool "Collect extra exception info"
+	depends on EXCEPTION_DEBUG
+	help
+	  This option enables the collection of extra information, such as
+	  register state, when a fault occurs. This information can be useful
+	  to collect for post-mortem analysis and debug of issues.
+
 config RISCV_PMP
 	bool "RISC-V PMP Support"
 	select THREAD_STACK_INFO

arch/riscv/core/isr.S

@@ -450,10 +450,15 @@ do_fault:
 	STORE_CALLEE_SAVED()		;
 	mv a2, sp
 
+#ifdef CONFIG_EXTRA_EXCEPTION_INFO
+	/* Store csf's addr into esf (a1 still holds the pointer to the esf at this point) */
+	sr a2 __struct_arch_esf_csf_OFFSET(a1)
+#endif /* CONFIG_EXTRA_EXCEPTION_INFO */
+
 	tail z_riscv_fatal_error_csf
 #else
 	tail z_riscv_fatal_error
-#endif
+#endif /* CONFIG_EXCEPTION_DEBUG */
 
 #if defined(CONFIG_IRQ_OFFLOAD)
 do_irq_offload:

arch/riscv/core/offsets/offsets.c

@@ -118,6 +118,10 @@ GEN_OFFSET_STRUCT(arch_esf, s0);
 GEN_OFFSET_STRUCT(arch_esf, sp);
 #endif
 
+#ifdef CONFIG_EXTRA_EXCEPTION_INFO
+GEN_OFFSET_STRUCT(arch_esf, csf);
+#endif /* CONFIG_EXTRA_EXCEPTION_INFO */
+
 #if defined(CONFIG_RISCV_SOC_CONTEXT_SAVE)
 GEN_OFFSET_STRUCT(arch_esf, soc_context);
 #endif

doc/releases/release-notes-4.0.rst

@@ -64,6 +64,13 @@ Architectures
   * The stack traces upon fatal exception now prints the address of stack pointer (sp) or frame
     pointer (fp) depending on the build configuration.
 
+  * When :kconfig:option:`CONFIG_EXTRA_EXCEPTION_INFO` is enabled, the exception stack frame (arch_esf)
+    has an additional field ``csf`` that points to the callee-saved-registers upon an fatal error,
+    which can be accessed in :c:func:`k_sys_fatal_error_handler` by ``esf->csf``.
+
+    * For SoCs that select `RISCV_SOC_HAS_ISR_STACKING`, the `SOC_ISR_STACKING_ESF_DECLARE` has to
+      include the `csf` member, otherwise the build would fail.
+
 * Xtensa
 
 * x86

include/zephyr/arch/riscv/exception.h

@@ -45,6 +45,12 @@ struct soc_esf {
 };
 #endif
 
+#ifdef CONFIG_EXTRA_EXCEPTION_INFO
+/* Forward declaration */
+struct _callee_saved;
+typedef struct _callee_saved _callee_saved_t;
+#endif /* CONFIG_EXTRA_EXCEPTION_INFO */
+
 #if defined(CONFIG_RISCV_SOC_HAS_ISR_STACKING)
 	SOC_ISR_STACKING_ESF_DECLARE;
 #else
@@ -81,6 +87,10 @@ struct arch_esf {
 	unsigned long sp;		/* preserved (user or kernel) stack pointer */
 #endif
 
+#ifdef CONFIG_EXTRA_EXCEPTION_INFO
+	_callee_saved_t *csf;		/* pointer to callee-saved-registers */
+#endif /* CONFIG_EXTRA_EXCEPTION_INFO */
+
 #ifdef CONFIG_RISCV_SOC_CONTEXT_SAVE
 	struct soc_esf soc_context;
 #endif