Setup the stack for an init task

Bootstrap an init task which holds at least the initial stack that is to
be used when setting up the registers (such as `sp` and `tp`).

In order to guarantee that the stack and the rest of the registers are
set up correctly, this commit also provides a raw implementation of a
`printk` function.

Moreover, this commit also adds an argument that must be passed to
`start_kernel`, which is the pointer to the embedded `fdt` blob. This
argument will be used by later work so to fetch, at least, the base
address for the initial ram disk. This was also used to test that the
stack was working as expected.

Signed-off-by: Miquel Sabaté Solà <mikisabate@gmail.com>

17 files changed, 281 insertions, 24 deletions

diff --git a/Makefile b/Makefile
index dae4248..b793ba8 100644
--- a/Makefile
+++ b/Makefile
@@ -18,16 +18,15 @@ endif
 # no `-mcpu`, no `-mtune`, no funny business.
 
 CC    = $(CROSS_COMPILE)gcc$(CC_SUFFIX)
-AS    = $(CROSS_COMPILE)as$(CC_SUFFIX)
 LD    = $(CROSS_COMPILE)ld
 QEMU ?= qemu-system-riscv64
 
 ISA     ?= rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd_zba_zbb
-ASFLAGS  = -march=$(ISA) -mabi=lp64d
+ASFLAGS  = -march=$(ISA) -mabi=lp64d -mcmodel=medany
 CCFLAGS  = $(ASFLAGS) -Iinclude/
 CCFLAGS += -Werror -Wpedantic -Wall -Wextra -Wcast-align -Wcast-qual -Winit-self \
            -Wmissing-include-dirs -Wredundant-decls -Wshadow -Wsign-conversion \
-           -Wswitch-default -Wundef -Wunreachable-code -Wmissing-noreturn \
+           -Wswitch-default -Wundef -Wunreachable-code \
            -nostdinc -nostdlib -std=gnu17
 LDFLAGS  = -Iinclude/ -static -melf64lriscv -z noexecstack
 USRFLAGS = -static -melf64lriscv
@@ -48,6 +47,7 @@ ifeq ($(strip $(DEBUG)),)
 	CCFLAGS += -O3
 	QEMU_FLAGS += -nographic
 else
+	ASFLAGS += -g
 	CCFLAGS += -g
 	QEMU_FLAGS += -s -S
 endif
@@ -55,7 +55,7 @@ endif
 ##
 # Paths
 
-SRC    = $(wildcard kernel/head.S kernel/*.c)
+SRC    = $(filter-out kernel/fbos.ld.S, $(wildcard kernel/*.S kernel/*.c))
 OBJ    = $(patsubst %.c,%.o,$(patsubst %.S,%.o,$(SRC)))
 LINKER = kernel/fbos.ld
 KRNL   = fbos
@@ -83,7 +83,7 @@ $(KRNL): $(OBJ) $(LINKER).S
 
 .S.o:
 	$(E) "	CC	" $(*F)
-	$(Q) $(CC) $(CCFLAGS) -c $< -o $@
+	$(Q) $(CC) $(CCFLAGS) -D__ASSEMBLY__ -c $< -o $@
 
 ##
 # User space
@@ -94,8 +94,8 @@ usr: $(USR)
 	$(Q) find usr/bin/ -type f -executable | cpio -o --quiet -H newc > $(INIT)
 
 usr/src/%.o: usr/src/%.S
-	$(E) "	AS	" $(basename $@)
-	$(Q) $(AS) $(ASFLAGS) -c $< -o $@
+	$(E) "	CC	" $(basename $@)
+	$(Q) $(CC) $(ASFLAGS) -D__ASSEMBLY__ -c $< -o $@
 
 usr/bin/%: usr/src/%.o
 	$(Q) mkdir -p usr/bin/
diff --git a/README.md b/README.md
index 8ce947f..a7f8799 100644
--- a/README.md
+++ b/README.md
@@ -104,6 +104,19 @@ $ make gdb GDB_EXTRA_FLAGS="-tui"
 
 And now you have started a GDB session with a nice TUI interface.
 
+## Requirements
+
+We do not want to support a myriad of different scenarios, but we want to keep
+things simple. Hence, here there are some limitations/requirements:
+
+- You need a recent enough OpenSBI running on your firmware. I have tested this
+  on a QEMU which has OpenSBI v1.5, but any firmware that implements a Runtime
+  SBI version of 2.0 should be fine.
+- You are supposed to pass an `initrd` always. This kernel will not try to
+  magically come up with a made up file system or try to fetch something from an
+  existing one. An `initrd` is already provided for you on the default `make`
+  target, and that's what you are supposed to be passing to the kernel.
+
 ## Special thanks to
 
 SUSE for organizing [Hack Week 24](https://hackweek.opensuse.org/24/projects).
diff --git a/include/fbos/compiler.h b/include/fbos/compiler.h
index edf24d1..5e61ebd 100644
--- a/include/fbos/compiler.h
+++ b/include/fbos/compiler.h
@@ -1,9 +1,30 @@
 #ifndef __FBOS_COMPILER_H
 #define __FBOS_COMPILER_H
 
+/*
+ * Nicer looking versions of compiler attributes.
+ */
+
 #define __noreturn __attribute__((__noreturn__))
 
+/*
+ * Compiler attributes specific to linker sections.
+ */
+
 #define __section(s) __attribute__((__section__(s)))
-#define __kernel __section(".text.kernel")
+#define __kernel __section(".kernel.text")
+
+/*
+ * Multiple aliases for 64-bit integers which have their definition on the
+ * standard library.
+ */
+
+typedef long ssize_t;
+typedef unsigned long size_t;
+typedef unsigned long uint64_t;
+typedef unsigned long uintptr_t;
+
+// Helpful macro when prototyping.
+#define __unused(x) (void)x
 
 #endif /* __FBOS_COMPILER_H */
diff --git a/include/fbos/dt.h b/include/fbos/dt.h
new file mode 100644
index 0000000..39d8381
--- /dev/null
+++ b/include/fbos/dt.h
@@ -0,0 +1,8 @@
+#ifndef __FBOS_DT_H_
+#define __FBOS_DT_H_
+
+#include <fbos/compiler.h>
+
+void parse_dtb(uint64_t *dtb);
+
+#endif // __FBOS_DT_H_
diff --git a/include/fbos/init.h b/include/fbos/init.h
index 1eeb33a..a919405 100644
--- a/include/fbos/init.h
+++ b/include/fbos/init.h
@@ -1,8 +1,11 @@
-#ifndef FBOS_INIT_H
-#define FBOS_INIT_H
+#ifndef __FBOS_INIT_H
+#define __FBOS_INIT_H
 
 #include <fbos/compiler.h>
 
-extern __noreturn __kernel void start_kernel(void);
+extern struct task_struct init_task;
 
-#endif /* FBOS_INIT_H */
+// The entry point for the kernel.
+__noreturn __kernel void start_kernel(uintptr_t *dtb);
+
+#endif /* __FBOS_INIT_H */
diff --git a/include/fbos/mm.h b/include/fbos/mm.h
index 2b80076..25d18ce 100644
--- a/include/fbos/mm.h
+++ b/include/fbos/mm.h
@@ -1,5 +1,5 @@
-#ifndef FBOS_MM_H
-#define FBOS_MM_H
+#ifndef __FBOS_MM_H
+#define __FBOS_MM_H
 
 /*
  * Page = 4KB.
@@ -7,10 +7,17 @@
 #define PAGE_SIZE 0x1000
 
 /*
+ * Initial size of the thread, which coincides with the size of the stack for a
+ * given thread.
+ */
+#define THREAD_SIZE_ORDER 2
+#define THREAD_SIZE (PAGE_SIZE << THREAD_SIZE_ORDER)
+
+/*
  * The code will be linked to start at the first page, which will have a given
  * offset.
  */
 #define PAGE_OFFSET 0x80200000
 #define LINK_ADDR PAGE_OFFSET
 
-#endif /* FBOS_MM_H */
+#endif /* __FBOS_MM_H */
diff --git a/include/fbos/printk.h b/include/fbos/printk.h
new file mode 100644
index 0000000..2c82eda
--- /dev/null
+++ b/include/fbos/printk.h
@@ -0,0 +1,7 @@
+#ifndef __FBOS_PRINTK_H_
+#define __FBOS_PRINTK_H_
+
+extern void die(const char *const message);
+extern void printk(const char *const message);
+
+#endif // __FBOS_PRINTK_H_
diff --git a/include/fbos/sbi.h b/include/fbos/sbi.h
new file mode 100644
index 0000000..a6b893d
--- /dev/null
+++ b/include/fbos/sbi.h
@@ -0,0 +1,35 @@
+#ifndef __FBOS_SBI_H_
+#define __FBOS_SBI_H_
+
+// TODO
+/* SBI_SUCCESS 0 Completed successfully */
+/* SBI_ERR_FAILED -1 Failed */
+/* SBI_ERR_NOT_SUPPORTED -2 Not supported */
+/* SBI_ERR_INVALID_PARAM -3 Invalid parameter(s) */
+/* SBI_ERR_DENIED -4 Denied or not allowed */
+/* SBI_ERR_INVALID_ADDRESS -5 Invalid address(s) */
+/* SBI_ERR_ALREADY_AVAILABLE -6 Already available */
+/* SBI_ERR_ALREADY_STARTED -7 Already started */
+/* SBI_ERR_ALREADY_STOPPED -8 Already stopped */
+/* SBI_ERR_NO_SHMEM -9 Shared memory not available */
+
+enum sbi_ext {
+	DBCN_EXT = 0x4442434E,
+};
+
+enum dbcn_actions {
+	DBCN_WRITE = 0x00,
+};
+
+struct sbi_ret {
+	long error;
+	long value;
+};
+
+extern struct sbi_ret __sbi_ecall(unsigned long arg0, unsigned long arg1, unsigned long arg2,
+								  unsigned long arg3, unsigned long arg4, unsigned long arg5,
+								  int fid, int ext);
+
+#define sbi_ecall2(ext, fid, arg0, arg1) __sbi_ecall(arg0, arg1, 0, 0, 0, 0, fid, ext)
+
+#endif // __FBOS_SBI_H_
diff --git a/include/fbos/sched.h b/include/fbos/sched.h
new file mode 100644
index 0000000..626f72f
--- /dev/null
+++ b/include/fbos/sched.h
@@ -0,0 +1,8 @@
+#ifndef __FBOS_SCHED_H_
+#define __FBOS_SCHED_H_
+
+struct task_struct {
+	void *stack;
+};
+
+#endif // __FBOS_SCHED_H_
diff --git a/include/fbos/string.h b/include/fbos/string.h
new file mode 100644
index 0000000..c3da266
--- /dev/null
+++ b/include/fbos/string.h
@@ -0,0 +1,8 @@
+#ifndef __FBOS_STRING_H_
+#define __FBOS_STRING_H_
+
+#include <fbos/compiler.h>
+
+extern size_t strlen(const char *);
+
+#endif // __FBOS_STRING_H_
diff --git a/kernel/dt.c b/kernel/dt.c
new file mode 100644
index 0000000..55fa970
--- /dev/null
+++ b/kernel/dt.c
@@ -0,0 +1,6 @@
+#include <fbos/dt.h>
+
+__kernel void parse_dtb(uint64_t *dtb)
+{
+	__unused(dtb);
+}
diff --git a/kernel/fbos.ld.S b/kernel/fbos.ld.S
index f288161..f74d90f 100644
--- a/kernel/fbos.ld.S
+++ b/kernel/fbos.ld.S
@@ -3,15 +3,15 @@
 SECTIONS {
 	// Ensure that the image starts at the very exact address SBI expects it to.
 	. = LINK_ADDR;
+	_start = .;
+	. = ALIGN(PAGE_SIZE);
 
 	// You would usually want to separate the head section into its own thing
 	// instead of clumping it into the main `.text` one. Well, I'm no expert on
 	// linker configuration, so patches are welcome :)
 	.text : {
-		// The very first thing has to be the `_start` function, which is where
-		// SBI will jump into. Afterwards comes the rest of `.text.head`.
-		_start = .;
-		*(.text.head)
+		_text = .;
+		*(.head.text)
 
 		// Aligning it to a full page is maybe a bit too much considering how
 		// small `.text.head` really is. I just saw this same thing on the Linux
@@ -23,7 +23,7 @@ SECTIONS {
 		// that large. That is, we put first the very core of the kernel, and
 		// the rest can go wherever.
 		__kernel_text_start = .;
-		*(.text.kernel)
+		*(.kernel.text)
 		__kernel_text_end = .;
 
 		// And the rest.
@@ -49,4 +49,6 @@ SECTIONS {
 	.bss : {
     	*(.bss)
 	}
+
+	_end = .;
 }
diff --git a/kernel/head.S b/kernel/head.S
index ba155cb..de7852b 100644
--- a/kernel/head.S
+++ b/kernel/head.S
@@ -1,6 +1,55 @@
+#include <fbos/mm.h>
+
 .global _start
-.section .text.head
+.section .head.text
 
 _start:
-	// TODO
-	call start_kernel
+	// Mask all interrupts
+	csrw sie, zero
+	csrw sip, zero
+
+	// Flush the instruction cache
+	fence.i
+
+	// Reset all registers except ra, a0, a1.
+	li sp, 0
+	li gp, 0
+	li tp, 0
+	li t0, 0
+	li t1, 0
+	li t2, 0
+	li s0, 0
+	li s1, 0
+	li a2, 0
+	li a3, 0
+	li a4, 0
+	li a5, 0
+	li a6, 0
+	li a7, 0
+	li s2, 0
+	li s3, 0
+	li s4, 0
+	li s5, 0
+	li s6, 0
+	li s7, 0
+	li s8, 0
+	li s9, 0
+	li s10, 0
+	li s11, 0
+	li t3, 0
+	li t4, 0
+	li t5, 0
+	li t6, 0
+	csrw sscratch, 0
+
+	// Point tp and sp to the init task.
+	la tp, init_task
+	la sp, init_task + THREAD_SIZE
+
+	// The `start_kernel` function requires an argument to be passed, which is
+	// the pointer to the `fdt` blob. The bootloader puts this on the `a1`
+	// register, so let's move it now to `a0`.
+	mv a0, a1
+
+	// Start the kernel.
+	tail start_kernel
diff --git a/kernel/main.c b/kernel/main.c
index 325f858..07188b9 100644
--- a/kernel/main.c
+++ b/kernel/main.c
@@ -1,12 +1,28 @@
 #include <fbos/init.h>
+#include <fbos/printk.h>
+#include <fbos/mm.h>
+#include <fbos/sched.h>
+#include <fbos/dt.h>
+
+unsigned long init_stack[THREAD_SIZE / sizeof(unsigned long)];
+
+struct task_struct init_task = { .stack = init_stack };
 
 /*
  * This is the main entry point of the kernel after head.S is done. This
  * function can (and will) assume that everything has been reset and that we can
  * start the whole thing.
  */
-__noreturn __kernel void start_kernel(void)
+__noreturn __kernel void start_kernel(uintptr_t *dtb)
 {
+	// TODO: disable irqs, etc.
+
+	printk("Welcome to FizzBuzz OS!\n");
+
+	parse_dtb(dtb);
+
+	// TODO: reenable stuff
+
 	for (;;)
 		;
 }
diff --git a/kernel/printk.c b/kernel/printk.c
new file mode 100644
index 0000000..c0eedd9
--- /dev/null
+++ b/kernel/printk.c
@@ -0,0 +1,23 @@
+#include <fbos/printk.h>
+#include <fbos/string.h>
+#include <fbos/sbi.h>
+
+void __noreturn __kernel die(const char *const message)
+{
+	if (message) {
+		printk(message);
+	}
+
+	for (;;)
+		;
+}
+
+void __kernel printk(const char *const message)
+{
+	size_t len = strlen(message);
+	if (!len) {
+		return;
+	}
+
+	sbi_ecall2(DBCN_EXT, DBCN_WRITE, len, (unsigned long)message);
+}
diff --git a/kernel/sbi.c b/kernel/sbi.c
new file mode 100644
index 0000000..ff7556c
--- /dev/null
+++ b/kernel/sbi.c
@@ -0,0 +1,29 @@
+#include <fbos/compiler.h>
+#include <fbos/sbi.h>
+
+/* Implementation taken from the Linux kernel (6.12) */
+__kernel struct sbi_ret __sbi_ecall(unsigned long arg0, unsigned long arg1, unsigned long arg2,
+									unsigned long arg3, unsigned long arg4, unsigned long arg5,
+									int fid, int ext)
+{
+	struct sbi_ret ret;
+
+	register uintptr_t a0 asm("a0") = (uintptr_t)(arg0);
+	register uintptr_t a1 asm("a1") = (uintptr_t)(arg1);
+	register uintptr_t a2 asm("a2") = (uintptr_t)(arg2);
+	register uintptr_t a3 asm("a3") = (uintptr_t)(arg3);
+	register uintptr_t a4 asm("a4") = (uintptr_t)(arg4);
+	register uintptr_t a5 asm("a5") = (uintptr_t)(arg5);
+	register uintptr_t a6 asm("a6") = (uintptr_t)(fid);
+	register uintptr_t a7 asm("a7") = (uintptr_t)(ext);
+
+	asm volatile("ecall"
+				 : "+r"(a0), "+r"(a1)
+				 : "r"(a2), "r"(a3), "r"(a4), "r"(a5), "r"(a6), "r"(a7)
+				 : "memory");
+
+	ret.error = (long)a0;
+	ret.value = (long)a1;
+
+	return ret;
+}
diff --git a/kernel/strlen.S b/kernel/strlen.S
new file mode 100644
index 0000000..6185b94
--- /dev/null
+++ b/kernel/strlen.S
@@ -0,0 +1,22 @@
+.globl strlen
+.type strlen, @function
+
+/*
+ * Defined in include/fbos/string.h
+ *
+ *     size_t strlen(const char *str)
+ *
+ * Returns (a0): string length.
+ * Parameter (a0): string to measure.
+ * Clobbers: t0, t1.
+ */
+strlen:
+	mv t1, a0
+1:
+	lbu t0, 0(t1)
+	beqz t0, 2f
+	addi t1, t1, 1
+	j 1b
+2:
+	sub a0, t1, a0
+	ret