first commit

.clang-format

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+BasedOnStyle: WebKit
+IndentWidth: 4
+TabWidth: 4
+UseTab: Never
+AlignConsecutiveDeclarations: false
+AlignConsecutiveAssignments: false
+AlignTrailingComments: true
+ColumnLimit: 105
+BreakBeforeBraces: Attach
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: false
+AllowShortLambdasOnASingleLine: false
+PointerAlignment: Left
+SpaceBeforeParens: Never
+SpacesInParentheses: false
+SpacesInConditionalStatement: false
+SpacesInContainerLiterals: false
+SpaceAfterCStyleCast: false
+SpaceBeforeCpp11BracedList: false
+SpaceBeforeSquareBrackets: false
+SpacesBeforeTrailingComments: 2
+PenaltyBreakAssignment: 1000
+NamespaceIndentation: All
+

.gitignore

@@ -0,0 +1,3 @@
+compile_commands.json
+kernel.elf
+*.o

Makefile

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+TOOLCHAIN = riscv64-linux-gnu-
+QEMU=qemu-system-riscv64
+KERNEL = kernel.elf
+
+CFLAGS=-march=rv64i_zicsr_d -mabi=lp64d -Wall -Wextra -Werror -O2 -g -Iinclude
+LDFLAGS=-T linker.ld --no-dynamic-linker -m elf64lriscv -static -nostdlib
+QEMUFLAGS=-nographic -serial mon:stdio --no-reboot
+
+CFILES = $(shell find -L -type f -name "*.c")
+SFILES = $(shell find -L -type f -iname "*.s")
+OBJ := $(addsuffix .o,$(CFILES)) $(addsuffix .o,$(SFILES))
+
+$(KERNEL): $(OBJ) linker.ld
+	${TOOLCHAIN}ld $(LDFLAGS) -o $(KERNEL) $(OBJ)
+
+%.c.o: %.c
+	$(TOOLCHAIN)gcc $(CFLAGS) -o $@ -c $<
+
+%.s.o: %.s
+	$(TOOLCHAIN)gcc $(CFLAGS) -o $@ -c $<
+
+%.S.o: %.S
+	$(TOOLCHAIN)gcc $(CFLAGS) -o $@ -c $<
+
+run: $(KERNEL)
+	qemu-system-riscv64 $(QEMUFLAGS) -kernel $(KERNEL)
+
+run-debug: $(KERNEL)
+	$(QEMU) -monitor none -s -S -kernel $(KERNEL) &
+	$(TOOLCHAIN)gdb $(KERNEL) -q -ex "target remote :1234" -ex "b *_start" -ex "c" -tui
+	killall $(QEMU)
+
+clean:
+	@rm $(OBJ) $(KERNEL)

include/atomic_helpers.h

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+#ifndef ATOMIC_HELPERS_H
+#define ATOMIC_HELPERS_H
+
+#define csrr(reg)                                                                                       \
+    ({                                                                                                  \
+        uintptr_t temp;                                                                                 \
+        __asm__ volatile("csrr  %0, " #reg : "=r"(temp));                                               \
+        temp;                                                                                           \
+    })
+
+#define csrw(reg, val)                                                                                  \
+    ({                                                                                                  \
+        uintptr_t temp = (uintptr_t)val;                                                                \
+        __asm__ volatile("csrw " #reg ", %0" ::"r"(temp));                                              \
+    })
+
+#endif

linker.ld

@@ -0,0 +1,9 @@
+MEMORY {
+  rom (rwx) : ORIGIN = 0x80200000, LENGTH = 512K
+}
+
+SECTIONS {
+  .init : {
+    *(.init)
+  } > rom
+}

src/entry.s

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+.global _start
+.global sret_wrapper
+.extern main
+
+.section .bss
+stack: .space 4096
+stack_top:
+
+.section .init
+_start:
+    la sp, stack_top
+    mv s0, sp
+    call main
+    sret
+.loop:
+    wfi
+    j .loop

src/kernel.c

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+#include <stddef.h>
+#include <stdint.h>
+#include <string.h>
+#include "atomic_helpers.h"
+
+#define DEBUG_ECALL 0x4442434E
+
+typedef struct {
+    long error;
+    long value;
+} sbiret;
+
+void trap_handler();
+void trap_handler_wrapper() __attribute__((naked));
+
+void user_entry();
+
+unsigned long strlen(const char* str) {
+    unsigned long len = 0;
+    while(str[len] != '\0') {
+        len++;
+    }
+
+    return len;
+}
+
+int main() {
+    csrw(stvec, trap_handler_wrapper);
+    csrw(sepc, user_entry);
+
+    unsigned long sstatus = csrr(sstatus);
+    sstatus |= (1 << 5);
+    csrw(sstatus, sstatus);
+
+    // _start in entry.s will do the sret
+}
+
+void user_entry(void) {
+    while(1) {
+        __asm__ volatile("ecall");
+        __asm__ volatile("unimp");
+    }
+}
+
+__attribute__((always_inline)) 
+inline sbiret sbi_ecall6(unsigned long number, unsigned long arg0, unsigned long arg1, unsigned long arg2, unsigned long arg3, unsigned long arg4, unsigned long arg5) {
+    sbiret ret;
+
+    __asm__ volatile("mv a7, %[number]\n"
+                     "li a6, 0\n"
+                     "mv a0, %[arg0]\n"
+                     "mv a1, %[arg1]\n"
+                     "mv a2, %[arg2]\n"
+                     "mv a3, %[arg3]\n"
+                     "mv a4, %[arg4]\n"
+                     "mv a5, %[arg5]\n"
+                     "ecall\n"
+                     "mv %[err], a0\n"
+                     "mv %[val], a1\n"
+        : [err] "=r"(ret.error), [val] "=r"(ret.value)
+        : [number] "r"(number), [arg0] "r"(arg0), [arg1] "r"(arg1),
+[arg2] "r"(arg2),[arg3] "r"(arg3), [arg4] "r"(arg4),[arg5] "r"(arg5)  
+        : "a0", "a1", "a2", "a3", "a4", "a5", "a6", "a7");
+
+    return ret;
+}
+
+__attribute__((always_inline)) 
+inline sbiret sbi_ecall5(unsigned long number, unsigned long arg0, unsigned long arg1, unsigned long arg2, unsigned long arg3, unsigned long arg4) {
+    return sbi_ecall6(number, arg0, arg1, arg2, arg3, arg4, 0);
+}
+
+__attribute__((always_inline)) 
+inline sbiret sbi_ecall4(unsigned long number, unsigned long arg0, unsigned long arg1, unsigned long arg2, unsigned long arg3) {
+    return sbi_ecall5(number, arg0, arg1, arg2, arg3, 0);
+}
+
+__attribute__((always_inline)) 
+inline sbiret sbi_ecall3(unsigned long number, unsigned long arg0, unsigned long arg1, unsigned long arg2) {
+    return sbi_ecall4(number, arg0, arg1, arg2, 0);
+}
+
+__attribute__((always_inline)) 
+inline sbiret sbi_ecall2(unsigned long number, unsigned long arg0, unsigned long arg1) {
+    return sbi_ecall3(number, arg0, arg1, 0);
+}
+
+__attribute__((always_inline))
+inline sbiret sbi_ecall1(unsigned long number, unsigned long arg0) {
+    return sbi_ecall2(number, arg0, 0);
+}
+
+__attribute__((always_inline)) 
+inline sbiret sbi_ecall0(unsigned long number) {
+    return sbi_ecall1(number, 0);
+}
+
+sbiret console_debug_write(const char* str, unsigned long len) {
+    return sbi_ecall2(DEBUG_ECALL, len, (unsigned long)str);
+}
+
+int puts(const char* str) {
+    sbiret ret;
+    if((ret = console_debug_write(str, strlen(str))).error != 0) {
+        return -1;
+    }
+    
+    if((ret = console_debug_write("\n", 1)).error != 0) {
+        return -1;
+    }
+    return 0;
+}
+
+void trap_handler_wrapper() {
+    __asm__ volatile("call trap_handler\n"
+                     "sret");
+}
+
+void trap_handler(void) {
+    unsigned long sepc = csrr(sepc);
+    unsigned long scause = csrr(scause);
+    unsigned long sstatus = csrr(sstatus);
+
+    if(scause == 8) {
+        puts("hello world");
+
+        sepc += 4;
+        csrw(sepc, sepc);
+        goto exit;
+    }
+
+    if(((sstatus >> 8) & 1) == 0) {
+        sepc += 4;
+        puts("exception occured in usermode, skipping past");
+        csrw(sepc, sepc);
+        goto exit;
+    } else {
+        puts("exception occured in supervisor mode, panic");
+
+        while(1) {
+            __asm__ volatile("wfi");
+        }
+    }
+
+exit:
+    return;
+}