commit 8197d4ef5e42be2f6f69512c8af8289cacc56fe1
parent d8cc5466839979bde2cb1f00db5af6ba9794a238
Author: Brian Swetland <swetland@frotz.net>
Date: Sun, 15 May 2022 11:00:37 -0700
example: ex25-kernel
- initial mmu setup for kernel at va 0xC0000000
- mmu setup for userspace program
- userspace program start
- syscall handling (exit and putc)
Diffstat:
6 files changed, 220 insertions(+), 0 deletions(-)
diff --git a/example/ex25-kernel.c b/example/ex25-kernel.c
@@ -0,0 +1,169 @@
+// Copyright 2022, Brian Swetland <swetland@frotz.net>
+// Licensed under the Apache License, Version 2.0
+
+#include <hw/riscv.h>
+#include <hw/context.h>
+#include <hw/debug.h>
+#include <hw/intrinsics.h>
+
+#include <hw/platform.h>
+#include <hw/litex.h>
+
+#include <string.h>
+
+#include "ex25-syscalls.h"
+
+#define PAGE_SIZE 4096
+
+typedef uint32_t paddr_t;
+typedef uint32_t vaddr_t;
+
+// physical address to kernel virtual address
+static inline void* pa_to_kva(paddr_t pa) {
+ return (void*) (pa + (0xC0000000 - 0x40000000));
+}
+static inline paddr_t kva_to_pa(void *kva) {
+ return ((paddr_t) kva) - (0xC0000000 - 0x40000000);
+}
+
+// trivial physical page allocator
+paddr_t ppage_next = 0;
+
+paddr_t alloc_ppage(void) {
+ ppage_next -= PAGE_SIZE;
+ memset(pa_to_kva(ppage_next), 0, PAGE_SIZE);
+ return ppage_next;
+}
+
+#define PTE_PA(pte) (((pte) & 0xFFFFFC00) << 2)
+
+uint32_t* pdir = 0;
+
+#define MAP_USER_RW (PTE_D|PTE_A|PTE_U|PTE_R|PTE_W|PTE_V)
+#define MAP_USER_RX (PTE_D|PTE_A|PTE_U|PTE_R|PTE_X|PTE_V)
+#define MAP_USER_RWX (PTE_D|PTE_A|PTE_U|PTE_R|PTE_W|PTE_X|PTE_V)
+
+int map_page(vaddr_t va, paddr_t pa, uint32_t flags) {
+ uint32_t idx1 = va >> 22;
+ uint32_t idx0 = (va >> 12) & 0x3FF;
+ uint32_t *ptbl;
+
+ xprintf("map page va=0x%08x(%d,%d) -> pa=0x%08x\n", va, idx1, idx0, pa);
+
+ uint32_t pte = pdir[idx1];
+ if (pte & PTE_V) { // valid pgdir entry
+ if (pte & (PTE_X|PTE_W|PTE_R)) {
+ return -1; // 4MB megapage already here
+ }
+ ptbl = pa_to_kva(PTE_PA(pte));
+ xprintf("old ptbl pa=0x%08x va=%p\n", PTE_PA(pte), ptbl);
+ } else { // no entry, allocate a pagetable
+ uint32_t ptbl_pa = alloc_ppage();
+ ptbl = pa_to_kva(ptbl_pa);
+ xprintf("new ptbl pa=0x%08x va=%p\n", ptbl_pa, ptbl);
+ pdir[idx1] = (ptbl_pa >> 2) | PTE_V;
+ }
+
+ pte = ptbl[idx0];
+ if (pte & PTE_V) {
+ return -1; // 4KB page already here
+ }
+
+ ptbl[idx0] = ((pa & 0xFFFFF000) >> 2) | (flags & 0x3FF);
+
+ tlb_flush_all();
+
+ return 0;
+}
+
+// load general registers from eframe
+// save ef + sizeof(eframe_t) as kernel sp for next trap
+// enter user mode at ef->pc
+void user_mode_entry(eframe_t* ef);
+
+extern char __extra_start[];
+extern char __extra_end[];
+
+void start_user_program(void) {
+ vaddr_t user_start = 0x10000000;
+ vaddr_t user_stack = 0x10400000;
+
+ xprintf("user.bin %u bytes\n", (__extra_end - __extra_start));
+
+ // allocate 16KB (4 pages) for user text/data
+ map_page(user_start + 0*PAGE_SIZE, alloc_ppage(), MAP_USER_RWX);
+ map_page(user_start + 1*PAGE_SIZE, alloc_ppage(), MAP_USER_RWX);
+ map_page(user_start + 2*PAGE_SIZE, alloc_ppage(), MAP_USER_RWX);
+ map_page(user_start + 3*PAGE_SIZE, alloc_ppage(), MAP_USER_RWX);
+
+ // allocate a 4KB (1 page) for user stack
+ map_page(user_stack - 1*PAGE_SIZE, alloc_ppage(), MAP_USER_RW);
+
+ // allow S-MODE writes to U-MODE pages
+ csr_set(CSR_SSTATUS, SSTATUS_SUM);
+
+ // copy embedded user.bin into user memory
+ memcpy((void*) user_start, __extra_start, __extra_end - __extra_start);
+
+ csr_clr(CSR_SSTATUS, SSTATUS_SUM);
+
+ // allocate a kernel stack page
+ // setup an eframe with the initial user register state
+ eframe_t* ef = pa_to_kva(alloc_ppage() + PAGE_SIZE - sizeof(eframe_t));
+ ef->pc = user_start;
+ ef->sp = user_stack;
+
+ // set previous privilege mode to user (0)
+ csr_clr(CSR_SSTATUS, SSTATUS_SPP);
+
+ xprintf("\n[ starting user mode program ]\n\n");
+
+ user_mode_entry(ef);
+ // does not return
+}
+
+void start(void *memtop) {
+ xprintf("Example 25 - kernel\n\n");
+
+ // set trap vector to trap_entry() in trap-entry-single.S
+ // it will call exception_handler() or interrupt_handler()
+ csr_write(CSR_STVEC, (uintptr_t) trap_entry);
+
+ // start allocating from 2 pages down from top of ram
+ // since our boot stack starts at top of ram
+ // and the initial page directory is just below that
+ ppage_next = kva_to_pa(memtop) - 2 * PAGE_SIZE;
+ pdir = (void*) (memtop - 2 * PAGE_SIZE);
+
+ start_user_program();
+}
+
+void interrupt_handler(void) {
+}
+
+// if an exception occurs, dump register state and halt
+void exception_handler(eframe_t *ef) {
+ uint32_t cause = csr_read(CSR_SCAUSE);
+
+ if (cause == EXCn_ECALL_UMODE) {
+ switch (ef->t0) { // syscall number
+ case SYS_EXIT:
+ xprintf("\n[ user exit (status %d) ]\n", ef->a0);
+ for (;;) ;
+ case SYS_XPUTC:
+ xputc(ef->a0);
+ break;
+ default:
+ xprintf("\n[ invalid syscall %u ]\n", ef->t0);
+ for (;;) ;
+ }
+ ef->pc += 4;
+ return;
+ }
+
+ xprintf("\n** SUPERVISOR EXCEPTION **\n");
+ xprint_s_exception(ef);
+ xprintf("\nHALT\n");
+ for (;;) ;
+}
+
diff --git a/example/ex25-syscalls.S b/example/ex25-syscalls.S
@@ -0,0 +1,14 @@
+
+#include "ex25-syscalls.h"
+
+.globl exit
+exit:
+ li t0, SYS_EXIT
+ ecall
+ ret
+
+.globl xputc
+xputc:
+ li t0, SYS_XPUTC
+ ecall
+ ret
diff --git a/example/ex25-syscalls.h b/example/ex25-syscalls.h
@@ -0,0 +1,4 @@
+#pragma once
+
+#define SYS_EXIT 0
+#define SYS_XPUTC 1
diff --git a/example/ex25-user.c b/example/ex25-user.c
@@ -0,0 +1,15 @@
+// Copyright 2022, Brian Swetland <swetland@frotz.net>
+// Licensed under the Apache License, Version 2.0
+
+#include <hw/debug.h>
+
+void xputs(const char* s) {
+ while (*s) {
+ xputc(*s++);
+ }
+}
+
+void start(void) {
+ xprintf("Hello, User Mode!\n");
+ exit(42);
+}
diff --git a/project/ex25-kernel.app.mk b/project/ex25-kernel.app.mk
@@ -0,0 +1,10 @@
+
+MOD_NAME := ex25-kernel
+MOD_SRC := hw/src/start.mmu.S hw/src/trap-entry-dual-stack.S
+MOD_SRC += example/ex25-kernel.c
+MOD_SRC += hw/src/context-switch.S
+MOD_SRC += hw/src/debug-printf.c hw/src/debug-io.c hw/src/print-exception.c
+MOD_LIB := c
+MOD_EXT := out/ex25-user.bin
+MOD_LDSCRIPT := make/kernel.ram.ld
+include make/app.mk
diff --git a/project/ex25-user.app.mk b/project/ex25-user.app.mk
@@ -0,0 +1,8 @@
+
+MOD_NAME := ex25-user
+MOD_SRC := hw/src/start.S
+MOD_SRC += example/ex25-user.c example/ex25-syscalls.S
+MOD_SRC += hw/src/debug-printf.c
+MOD_LIB := c
+MOD_LDSCRIPT := make/user.ram.ld
+include make/app.mk
