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 <[email protected]>

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  = $(ASFLAGS) -Iinclude/
 CCFLAGS += -Werror -Wpedantic -Wall -Wextra -Wcast-align -Wcast-qual -Winit-self \
            -Wmissing-include-dirs -Wredundant-decls -Wshadow -Wsign-conversion \
-           -Wswitch-default -Wundef -Wunreachable-code -Wmissing-noreturn \
+           -Wswitch-default -Wundef -Wunreachable-code \
            -nostdinc -nostdlib -std=gnu17
 LDFLAGS  = -Iinclude/ -static -melf64lriscv -z noexecstack
 USRFLAGS = -static -melf64lriscv
@@ -48,14 +47,15 @@ ifeq ($(strip $(DEBUG)),)
 	CCFLAGS += -O3
 	QEMU_FLAGS += -nographic
 else
+	ASFLAGS += -g
 	CCFLAGS += -g
 	QEMU_FLAGS += -s -S
 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/

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).

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 */

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 */

include/fbos/mm.h

@@ -1,16 +1,23 @@
-#ifndef FBOS_MM_H
-#define FBOS_MM_H
+#ifndef __FBOS_MM_H
+#define __FBOS_MM_H
 
 /*
  * Page = 4KB.
  */
 #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 */

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 = .;
 }

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

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 (;;)
 		;
 }