Bootstrap the project

For now the base layout has been defined and we have a binary that is
properly reached through openSBI.

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

13 files changed, 1095 insertions, 0 deletions

diff --git a/.clang-format b/.clang-format
new file mode 100644
index 0000000..d21d53a
--- /dev/null
+++ b/.clang-format
@@ -0,0 +1,109 @@
+# Modified from the Linux Kernel.
+---
+AccessModifierOffset: -4
+AlignAfterOpenBracket: Align
+AlignConsecutiveAssignments: false
+AlignConsecutiveDeclarations: false
+AlignEscapedNewlines: Left
+AlignOperands: true
+AlignTrailingComments: false
+AllowAllParametersOfDeclarationOnNextLine: false
+AllowShortBlocksOnASingleLine: false
+AllowShortCaseLabelsOnASingleLine: false
+AllowShortFunctionsOnASingleLine: None
+AllowShortIfStatementsOnASingleLine: false
+AllowShortLoopsOnASingleLine: false
+AlwaysBreakAfterDefinitionReturnType: None
+AlwaysBreakAfterReturnType: None
+AlwaysBreakBeforeMultilineStrings: false
+AlwaysBreakTemplateDeclarations: false
+BinPackArguments: true
+BinPackParameters: true
+BraceWrapping:
+  AfterClass: false
+  AfterControlStatement: false
+  AfterEnum: false
+  AfterFunction: true
+  AfterNamespace: true
+  AfterObjCDeclaration: false
+  AfterStruct: false
+  AfterUnion: false
+  AfterExternBlock: false
+  BeforeCatch: false
+  BeforeElse: false
+  IndentBraces: false
+  SplitEmptyFunction: true
+  SplitEmptyRecord: true
+  SplitEmptyNamespace: true
+BreakBeforeBinaryOperators: None
+BreakBeforeBraces: Custom
+BreakBeforeInheritanceComma: false
+BreakBeforeTernaryOperators: false
+BreakConstructorInitializersBeforeComma: false
+BreakConstructorInitializers: BeforeComma
+BreakAfterJavaFieldAnnotations: false
+BreakStringLiterals: false
+ColumnLimit: 100
+CommentPragmas: '^ IWYU pragma:'
+CompactNamespaces: false
+ConstructorInitializerAllOnOneLineOrOnePerLine: false
+ConstructorInitializerIndentWidth: 4
+ContinuationIndentWidth: 4
+Cpp11BracedListStyle: false
+DerivePointerAlignment: false
+DisableFormat: false
+ExperimentalAutoDetectBinPacking: false
+FixNamespaceComments: false
+
+IncludeBlocks: Preserve
+IncludeCategories:
+  - Regex: '.*'
+    Priority: 1
+IncludeIsMainRegex: '(Test)?$'
+IndentCaseLabels: false
+IndentGotoLabels: false
+IndentPPDirectives: None
+IndentWidth: 4
+IndentWrappedFunctionNames: false
+JavaScriptQuotes: Leave
+JavaScriptWrapImports: true
+KeepEmptyLinesAtTheStartOfBlocks: false
+MacroBlockBegin: ''
+MacroBlockEnd: ''
+MaxEmptyLinesToKeep: 1
+NamespaceIndentation: None
+ObjCBinPackProtocolList: Auto
+ObjCBlockIndentWidth: 4
+ObjCSpaceAfterProperty: true
+ObjCSpaceBeforeProtocolList: true
+
+PenaltyBreakAssignment: 10
+PenaltyBreakBeforeFirstCallParameter: 30
+PenaltyBreakComment: 10
+PenaltyBreakFirstLessLess: 0
+PenaltyBreakString: 10
+PenaltyExcessCharacter: 100
+PenaltyReturnTypeOnItsOwnLine: 60
+
+PointerAlignment: Right
+ReflowComments: false
+SortIncludes: false
+SortUsingDeclarations: false
+SpaceAfterCStyleCast: false
+SpaceAfterTemplateKeyword: true
+SpaceBeforeAssignmentOperators: true
+SpaceBeforeCtorInitializerColon: true
+SpaceBeforeInheritanceColon: true
+SpaceBeforeParens: ControlStatementsExceptForEachMacros
+SpaceBeforeRangeBasedForLoopColon: true
+SpaceInEmptyParentheses: false
+SpacesBeforeTrailingComments: 1
+SpacesInAngles: false
+SpacesInContainerLiterals: false
+SpacesInCStyleCastParentheses: false
+SpacesInParentheses: false
+SpacesInSquareBrackets: false
+Standard: Latest
+TabWidth: 4
+UseTab: Always
+...
diff --git a/.editorconfig b/.editorconfig
new file mode 100644
index 0000000..e7bd01a
--- /dev/null
+++ b/.editorconfig
@@ -0,0 +1,8 @@
+root = true
+
+[{*.{c,h,s,S},Makefile,Makefile.*}]
+charset = utf-8
+end_of_line = lf
+insert_final_newline = true
+indent_style = tab
+indent_size = 4
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..f1ccbff
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,7 @@
+*.o
+kernel/fbos.ld
+/fbos
+.cache
+
+# You can generate it with Bear: `$ bear -- make`.
+compile_commands.json
diff --git a/LICENSE b/LICENSE
new file mode 100644
index 0000000..f288702
--- /dev/null
+++ b/LICENSE
@@ -0,0 +1,674 @@
+                    GNU GENERAL PUBLIC LICENSE
+                       Version 3, 29 June 2007
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+
+  A patent license is "discriminatory" if it does not include within
+the scope of its coverage, prohibits the exercise of, or is
+conditioned on the non-exercise of one or more of the rights that are
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+or that patent license was granted, prior to 28 March 2007.
+
+  Nothing in this License shall be construed as excluding or limiting
+any implied license or other defenses to infringement that may
+otherwise be available to you under applicable patent law.
+
+  12. No Surrender of Others' Freedom.
+
+  If conditions are imposed on you (whether by court order, agreement or
+otherwise) that contradict the conditions of this License, they do not
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+
+  13. Use with the GNU Affero General Public License.
+
+  Notwithstanding any other provision of this License, you have
+permission to link or combine any covered work with a work licensed
+under version 3 of the GNU Affero General Public License into a single
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+
+  14. Revised Versions of this License.
+
+  The Free Software Foundation may publish revised and/or new versions of
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+be similar in spirit to the present version, but may differ in detail to
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+
+  Each version is given a distinguishing version number.  If the
+Program specifies that a certain numbered version of the GNU General
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+option of following the terms and conditions either of that numbered
+version or of any later version published by the Free Software
+Foundation.  If the Program does not specify a version number of the
+GNU General Public License, you may choose any version ever published
+by the Free Software Foundation.
+
+  If the Program specifies that a proxy can decide which future
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+to choose that version for the Program.
+
+  Later license versions may give you additional or different
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+
+  15. Disclaimer of Warranty.
+
+  THERE IS NO WARRANTY FOR THE PROGRAM, TO THE EXTENT PERMITTED BY
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+  16. Limitation of Liability.
+
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+SUCH DAMAGES.
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+  17. Interpretation of Sections 15 and 16.
+
+  If the disclaimer of warranty and limitation of liability provided
+above cannot be given local legal effect according to their terms,
+reviewing courts shall apply local law that most closely approximates
+an absolute waiver of all civil liability in connection with the
+Program, unless a warranty or assumption of liability accompanies a
+copy of the Program in return for a fee.
+
+                     END OF TERMS AND CONDITIONS
+
+            How to Apply These Terms to Your New Programs
+
+  If you develop a new program, and you want it to be of the greatest
+possible use to the public, the best way to achieve this is to make it
+free software which everyone can redistribute and change under these terms.
+
+  To do so, attach the following notices to the program.  It is safest
+to attach them to the start of each source file to most effectively
+state the exclusion of warranty; and each file should have at least
+the "copyright" line and a pointer to where the full notice is found.
+
+    <one line to give the program's name and a brief idea of what it does.>
+    Copyright (C) <year>  <name of author>
+
+    This program is free software: you can redistribute it and/or modify
+    it under the terms of the GNU General Public License as published by
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see <https://www.gnu.org/licenses/>.
+
+Also add information on how to contact you by electronic and paper mail.
+
+  If the program does terminal interaction, make it output a short
+notice like this when it starts in an interactive mode:
+
+    <program>  Copyright (C) <year>  <name of author>
+    This program comes with ABSOLUTELY NO WARRANTY; for details type `show w'.
+    This is free software, and you are welcome to redistribute it
+    under certain conditions; type `show c' for details.
+
+The hypothetical commands `show w' and `show c' should show the appropriate
+parts of the General Public License.  Of course, your program's commands
+might be different; for a GUI interface, you would use an "about box".
+
+  You should also get your employer (if you work as a programmer) or school,
+if any, to sign a "copyright disclaimer" for the program, if necessary.
+For more information on this, and how to apply and follow the GNU GPL, see
+<https://www.gnu.org/licenses/>.
+
+  The GNU General Public License does not permit incorporating your program
+into proprietary programs.  If your program is a subroutine library, you
+may consider it more useful to permit linking proprietary applications with
+the library.  If this is what you want to do, use the GNU Lesser General
+Public License instead of this License.  But first, please read
+<https://www.gnu.org/licenses/why-not-lgpl.html>.
diff --git a/Makefile b/Makefile
new file mode 100644
index 0000000..f969a1c
--- /dev/null
+++ b/Makefile
@@ -0,0 +1,90 @@
+##
+# By default everything is silent. If you want to change this behavior, simply
+# assign V=1 when calling make.
+
+V =
+ifeq ($(strip $(V)),)
+	E = @echo
+	Q = @
+else
+	E = @\#
+	Q =
+endif
+
+##
+# Compile options. You can use CROSS_COMPILE just like on the Linux Kernel. You
+# can also determine the ISA to be used, which defaults to my VisionFive2 board.
+# I did not go too much into the rabbit hole of platform-specific flags. Hence
+# no `-mcpu`, no `-mtune`, no funny business.
+
+CC   = $(CROSS_COMPILE)gcc
+LD   = $(CROSS_COMPILE)ld
+QEMU ?= qemu-system-riscv64
+
+ISA        ?= rv64imafdc_zicntr_zicsr_zifencei_zihpm_zca_zcd_zba_zbb
+CCFLAGS     = -march=$(ISA) -Iinclude/ -mabi=lp64
+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
+LDFLAGS     = -Iinclude/ -static -nostdlib -melf64lriscv -z noexecstack
+QEMU_FLAGS ?=
+QEMU_BIOS  ?=
+
+##
+# You can pass an optional `DEBUG` variable to manipulate the build type. This
+# will also affect how QEMU is run.
+
+DEBUG =
+ifeq ($(strip $(DEBUG)),)
+	CCFLAGS += -O3
+else
+	CCFLAGS += -g
+	QEMU_FLAGS += -s -S
+endif
+
+##
+# Paths.
+
+SRC    = $(wildcard kernel/head.S kernel/*.c)
+OBJ    = $(patsubst %.c,%.o,$(patsubst %.S,%.o,$(SRC)))
+LINKER = kernel/fbos.ld
+KRNL   = fbos
+
+LDFLAGS += -T $(LINKER)
+
+##
+# Targets
+
+.PHONY: all
+all: clean $(KRNL)
+
+.PHONY: $(KRNL)
+$(KRNL): $(OBJ) $(LINKER).S
+	$(Q) $(CC) -E $(LINKER).S -Iinclude/ -o $(LINKER)
+
+	$(E) "	LD	" $@
+	$(Q) $(LD) $(LDFLAGS) $(OBJ) -o $(KRNL)
+
+.c.o:
+	$(E) "	CC	" $(*F)
+	$(Q) $(CC) $(CCFLAGS) -c $< -o $@
+
+.S.o:
+	$(E) "	CC	" $(*F)
+	$(Q) $(CC) $(CCFLAGS) -c $< -o $@
+
+.PHONY: qemu
+qemu:
+ifeq ($(strip $(QEMU_BIOS)),)
+	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -kernel $(KRNL) -nographic
+else
+	$(Q) $(QEMU) $(QEMU_FLAGS) -machine virt -bios $(QEMU_BIOS) -kernel $(KRNL) -nographic
+endif
+
+.PHONY: gdb
+gdb:
+	$(Q) gdb --command utils/init.gdb
+
+.PHONY: clean
+clean:
+	$(Q) rm -f $(OBJ) $(KRNL) $(LINKER)
diff --git a/README.md b/README.md
new file mode 100644
index 0000000..532584b
--- /dev/null
+++ b/README.md
@@ -0,0 +1,101 @@
+A RISC-V operating system devoted to running fizz/buzz processes.
+
+FizzBuzz OS (or just `fbos`) is an excuse to better grasp the fundamentals of
+low level RISC-V machines. In practice, this is a small Operating System kernel
+that is able to launch three processes: one that simply prints "Fizz", another
+that prints "Buzz", and a final third which prints "FizzBuzz". These processes
+are unaware of each other and it's up to the kernel to schedule them by using
+the timer interrupts as given by openSBI (`fizz` on % 3 seconds, `buzz` on % 5
+seconds, and `fizzbuzz` on % 15 seconds).
+
+This kernel provides just one system call, `write`, which allows any program to
+pass the string to be written into the serial port.
+
+## Build
+
+In order to build `fbos` you only need GCC. That being said, if you are not on a
+RISC-V system you will also need cross compilation tools for it (check your
+distribution for this, on openSUSE I simply installed `cross-riscv64-gcc14`).
+With that installed, simply set the `CROSS_COMPILE` environment variable as
+you'd do for building the kernel (e.g. in my case, using openSUSE, I set it to
+`riscv64-suse-linux-`). After all of that, just:
+
+```
+$ make
+```
+
+And that's basically it! The build process should've produced a `fbos` binary
+sitting at the root of the project.
+
+### Verbose output
+
+You will notice that the output is suspiciously quiet. You can go back into
+verbose output by passing `V=1` to any make target. Hence, for the build step,
+you can do something like:
+
+```
+$ make V=1
+```
+
+### Debug mode
+
+By default the kernel will be built with optimizations on. You can disable this
+by passing the `DEBUG=1` option to any make target. This will also come in handy
+whenever you'd want to debug the kernel via QEMU+GDB. Read more on this below.
+
+## Test
+### QEMU
+
+The easiest way to test the `fbos` binary from the build step is with QEMU. That
+being said, this kernel only supports the bootflow of openSBI `fw_dynamic`. If
+you have a recent enough QEMU, then that's not a problem and you can skip the
+next step. Otherwise you might need to specify the BIOS firmware that you'll
+have to compile from openSBI. If you are on that side, don't worry, it's not
+that hard:
+
+```
+$ git clone https://github.com/riscv-software-src/opensbi.git
+$ cd opensbi
+$ make ARCH=riscv CROSS_COMPILE=<your-cross-compile> PLATFORM=generic
+# => build/platform/generic/firmware/fw_dynamic.bin
+```
+
+With that simply set the `QEMU_BIOS` environment variable with the full path of
+the resulting `fw_dynamic.bin` file.
+
+Now make sure that you have the version that is able to emulate a RISC-V system.
+After that, simply run:
+
+```
+$ make qemu
+```
+
+The `qemu` target can be paired with the `DEBUG` parameter that you can pass to
+make. Hence, you can also call it like so:
+
+```
+$ make qemu DEBUG=1
+```
+
+This will make QEMU wait for a GDB connection. On another terminal then type:
+
+```
+$ make gdb
+```
+
+Now you have a debugging session for this kernel. If you want to skip ahead,
+notice that the kernel starts at `PAGE_OFFSET` (see `include/fbos/mm.h`). Hence,
+upon starting the GDB session you can simply type:
+
+```
+# 0x80200000 is the current value of PAGE_OFFSET.
+(gdb) break *0x80200000
+(gdb) continue
+```
+
+From there you are already out of firmware code and right into the kernel.
+
+## Special thanks to
+
+SUSE for organizing [Hack Week 24](https://hackweek.opensuse.org/24/projects).
+This project was mainly developed during this time.
diff --git a/include/fbos/compiler.h b/include/fbos/compiler.h
new file mode 100644
index 0000000..edf24d1
--- /dev/null
+++ b/include/fbos/compiler.h
@@ -0,0 +1,9 @@
+#ifndef __FBOS_COMPILER_H
+#define __FBOS_COMPILER_H
+
+#define __noreturn __attribute__((__noreturn__))
+
+#define __section(s) __attribute__((__section__(s)))
+#define __kernel __section(".text.kernel")
+
+#endif /* __FBOS_COMPILER_H */
diff --git a/include/fbos/init.h b/include/fbos/init.h
new file mode 100644
index 0000000..1eeb33a
--- /dev/null
+++ b/include/fbos/init.h
@@ -0,0 +1,8 @@
+#ifndef FBOS_INIT_H
+#define FBOS_INIT_H
+
+#include <fbos/compiler.h>
+
+extern __noreturn __kernel void start_kernel(void);
+
+#endif /* FBOS_INIT_H */
diff --git a/include/fbos/mm.h b/include/fbos/mm.h
new file mode 100644
index 0000000..2b80076
--- /dev/null
+++ b/include/fbos/mm.h
@@ -0,0 +1,16 @@
+#ifndef FBOS_MM_H
+#define FBOS_MM_H
+
+/*
+ * Page = 4KB.
+ */
+#define PAGE_SIZE 0x1000
+
+/*
+ * 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 */
diff --git a/kernel/fbos.ld.S b/kernel/fbos.ld.S
new file mode 100644
index 0000000..f288161
--- /dev/null
+++ b/kernel/fbos.ld.S
@@ -0,0 +1,52 @@
+#include <fbos/mm.h>
+
+SECTIONS {
+	// Ensure that the image starts at the very exact address SBI expects it to.
+	. = LINK_ADDR;
+
+	// 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)
+
+		// 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
+		// Kernel and it felt clean.
+		. = ALIGN(PAGE_SIZE);
+
+		// From now on the rest of the `.text` could just be a continuation, but
+		// I further split it into `.text.kernel` so the jump from head isn't
+		// 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_end = .;
+
+		// And the rest.
+    	*(.text)
+	}
+
+	// In total we would reserve two full blown pages for the kernel code, which
+	// is definitely too much, but whatever, memory is cheap.
+	. = ALIGN(PAGE_SIZE);
+
+	.data : {
+    	*(.data)
+	}
+
+	. = ALIGN(8);
+
+	.rodata : {
+    	*(.rodata)
+	}
+
+	. = ALIGN(8);
+
+	.bss : {
+    	*(.bss)
+	}
+}
diff --git a/kernel/head.S b/kernel/head.S
new file mode 100644
index 0000000..ba155cb
--- /dev/null
+++ b/kernel/head.S
@@ -0,0 +1,6 @@
+.global _start
+.section .text.head
+
+_start:
+	// TODO
+	call start_kernel
diff --git a/kernel/main.c b/kernel/main.c
new file mode 100644
index 0000000..325f858
--- /dev/null
+++ b/kernel/main.c
@@ -0,0 +1,12 @@
+#include <fbos/init.h>
+
+/*
+ * 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)
+{
+	for (;;)
+		;
+}
diff --git a/utils/init.gdb b/utils/init.gdb
new file mode 100644
index 0000000..786e6fe
--- /dev/null
+++ b/utils/init.gdb
@@ -0,0 +1,3 @@
+set architecture riscv:rv64
+set disassemble-next-line on
+target remote :1234