arch: riscv: isr.S: restore s0 before jumping to z_riscv_fatal_error_csf

arch/riscv/core/isr.S

@@ -443,6 +443,12 @@ do_fault:
 1:	mv a1, sp
 
 #ifdef CONFIG_EXCEPTION_DEBUG
+	/*
+	 * Restore the s0 we saved early in ISR entry
+	 * so it shows up properly in the CSF.
+	 */
+	lr s0, __struct_arch_esf_s0_OFFSET(sp)
+
 	/* Allocate space for caller-saved registers on current thread stack */
 	addi sp, sp, -__callee_saved_t_SIZEOF
 

tests/arch/riscv/fatal/CMakeLists.txt

@@ -0,0 +1,8 @@
+# SPDX-License-Identifier: Apache-2.0
+
+cmake_minimum_required(VERSION 3.20.0)
+find_package(Zephyr REQUIRED HINTS $ENV{ZEPHYR_BASE})
+project(riscv_fatal)
+
+FILE(GLOB app_sources src/*.c)
+target_sources(app PRIVATE ${app_sources})

tests/arch/riscv/fatal/prj.conf

@@ -0,0 +1,2 @@
+CONFIG_TEST=y
+CONFIG_LOG=y

tests/arch/riscv/fatal/src/main.c

@@ -0,0 +1,62 @@
+/*
+ * Copyright (c) 2024 Meta Platforms
+ *
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+int main(void)
+{
+	__asm__(
+		/**
+		 * Load up a bunch of known values into registers
+		 * and expect them to show up in the core dump.
+		 * Value is register ABI name kinda spelled out,
+		 * followed by zeros to pad to 32 bits,
+		 * followed by FF00, followed by hex number of the register,
+		 * follwed by the "hex-coded-decismal" number of the register.
+		 */
+
+		/* "RA" -> "DA".  Kind of a stretch, but okay. */
+		"li x1, 0xDADA0000FF000101\n\t"
+
+		/* Skip stack pointer because it can mess stuff up. */
+		/* "li x2, 0\n\t" */
+
+		/* T0 -> D0.  Kinda close in pronunciation. */
+		"li x5, 0xD0FF0000FF000505\n\t"
+		"li x6, 0xD1FF0000FF000606\n\t"
+		"li x7, 0xD2FF0000FF000707\n\t"
+		/* S0 -> C0.  Kinda close in pronunciation. */
+		"li x8, 0xC0FF0000FF000808\n\t"
+		"li x9, 0xC1FF0000FF000909\n\t"
+		/* A0 -> A0.  Actual match! */
+		"li x10, 0xA0FF0000FF000A10\n\t"
+		"li x11, 0xA1FF0000FF000B11\n\t"
+		"li x12, 0xA2FF0000FF000C12\n\t"
+		"li x13, 0xA3FF0000FF000D13\n\t"
+		"li x14, 0xA4FF0000FF000E14\n\t"
+		"li x15, 0xA5FF0000FF000F15\n\t"
+		"li x16, 0xA6FF0000FF001016\n\t"
+		"li x17, 0xA7FF0000FF001117\n\t"
+		"li x18, 0xC2FF0000FF001218\n\t"
+		"li x19, 0xC3FF0000FF001319\n\t"
+		"li x20, 0xC4FF0000FF001420\n\t"
+		"li x21, 0xC5FF0000FF001521\n\t"
+		"li x22, 0xC6FF0000FF001622\n\t"
+		"li x23, 0xC7FF0000FF001723\n\t"
+		"li x24, 0xC8FF0000FF001824\n\t"
+		"li x25, 0xC9FF0000FF001925\n\t"
+		"li x26, 0xC10FF000FF001A26\n\t"
+		"li x27, 0xC11FF000FF001B27\n\t"
+		"li x28, 0xD3FF0000FF001C28\n\t"
+		"li x29, 0xD4FF0000FF001D29\n\t"
+		"li x30, 0xD5FF0000FF001E30\n\t"
+		"li x31, 0xD6FF0000FF001F31\n\t"
+		/* K_ERR_KERNEL_PANIC */
+		"li a0, 4\n\t"
+		/* RV_ECALL_RUNTIME_EXCEPT */
+		"li t0, 0\n\t"
+		"ecall\n");
+
+	return 0;
+}

tests/arch/riscv/fatal/testcase.yaml

@@ -0,0 +1,27 @@
+common:
+  ignore_faults: true
+  harness: console
+  ignore_qemu_crash: true
+  tags: kernel riscv
+  platform_allow:
+    - qemu_riscv64
+tests:
+  arch.riscv64.fatal:
+    harness_config:
+      type: multi_line
+      regex:
+        - "E:      a0: 0000000000000004    t0: 0000000000000000"
+        - "E:      a1: a1ff0000ff000b11    t1: d1ff0000ff000606"
+        - "E:      a2: a2ff0000ff000c12    t2: d2ff0000ff000707"
+        - "E:      a3: a3ff0000ff000d13    t3: d3ff0000ff001c28"
+        - "E:      a4: a4ff0000ff000e14    t4: d4ff0000ff001d29"
+        - "E:      a5: a5ff0000ff000f15    t5: d5ff0000ff001e30"
+        - "E:      a6: a6ff0000ff001016    t6: d6ff0000ff001f31"
+        - "E:      a7: a7ff0000ff001117"
+        - "E:      ra: dada0000ff000101"
+        - "E:      s0: c0ff0000ff000808    s6: c6ff0000ff001622"
+        - "E:      s1: c1ff0000ff000909    s7: c7ff0000ff001723"
+        - "E:      s2: c2ff0000ff001218    s8: c8ff0000ff001824"
+        - "E:      s3: c3ff0000ff001319    s9: c9ff0000ff001925"
+        - "E:      s4: c4ff0000ff001420   s10: c10ff000ff001a26"
+        - "E:      s5: c5ff0000ff001521   s11: c11ff000ff001b27"