syscall base

src/arch/x86/CMakeLists.txt

@@ -21,7 +21,9 @@ target_sources(kernel.elf PRIVATE
         paging.cpp
         kmain.cpp
         gdt.cpp
-        misc.cpp)
+        misc.cpp
+        syscalls.cpp
+        syscalls.asm)
 
 target_include_directories(kernel.elf PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
 

src/arch/x86/gdt.asm

@@ -60,13 +60,6 @@ gdt_data:
     db PRESENT | NOT_SYS | RW                   ; Access
     db GRAN_4K | SZ_32 | 0xF                    ; Flags & Limit (high, bits 16-19)
     db 0                                        ; Base (high, bits 24-31)
-global gdt_code_user:data
-gdt_code_user:
-    dd 0xFFFF                                   ; Limit & Base (low, bits 0-15)
-    db 0                                        ; Base (mid, bits 16-23)
-    db PRESENT | USER | NOT_SYS | EXEC | RW     ; Access
-    db GRAN_4K | LONG_MODE | 0xF                ; Flags & Limit (high, bits 16-19)
-    db 0                                        ; Base (high, bits 24-31)
 global gdt_data_user:data
 gdt_data_user:
     dd 0xFFFF                                   ; Limit & Base (low, bits 0-15)
@@ -74,6 +67,13 @@ gdt_data_user:
     db PRESENT | USER | NOT_SYS | RW            ; Access
     db GRAN_4K | SZ_32 | 0xF                    ; Flags & Limit (high, bits 16-19)
     db 0                                        ; Base (high, bits 24-31)
+global gdt_code_user:data
+gdt_code_user:
+    dd 0xFFFF                                   ; Limit & Base (low, bits 0-15)
+    db 0                                        ; Base (mid, bits 16-23)
+    db PRESENT | USER | NOT_SYS | EXEC | RW     ; Access
+    db GRAN_4K | LONG_MODE | 0xF                ; Flags & Limit (high, bits 16-19)
+    db 0                                        ; Base (high, bits 24-31)
 global gdt_tss:data
 gdt_tss:
     dq 0x00000000 ;TODO

src/arch/x86/kmain.cpp

@@ -15,6 +15,7 @@
 #include "misc.hpp"
 #include "mutex.hpp"
 #include "rand.h"
+#include "syscalls.hpp"
 #include "task.hpp"
 #include "timer.hpp"
 #include "tty.hpp"
@@ -187,8 +188,10 @@ void stress_tester() {
     remove_self();
 }
 
+
 void user_task() {
     while (true) {
+        asm("syscall");
         __builtin_ia32_pause();
     }
 }
@@ -231,6 +234,7 @@ void kmain() {
 
     new_ktask(ktask_main, "ktask_main");
     new_ktask(dummy_task, "dummy");
+    setup_syscalls();
     init_tasks();
     for (;;) {
         __asm__ __volatile__("hlt");

src/arch/x86/misc.hpp

@@ -54,6 +54,23 @@ static inline void irqrestore(unsigned long flags) {
         irqrestore(f);                       \
     }
 
+static inline void wrmsr(uint64_t msr, uint64_t value) {
+    uint32_t low = value & 0xFFFFFFFF;
+    uint32_t high = value >> 32;
+    asm volatile(
+            "wrmsr"
+            :
+            : "c"(msr), "a"(low), "d"(high));
+}
+
+static inline uint64_t rdmsr(uint64_t msr) {
+    uint32_t low, high;
+    asm volatile(
+            "rdmsr"
+            : "=a"(low), "=d"(high)
+            : "c"(msr));
+    return ((uint64_t) high << 32) | low;
+}
 
 char *itoa(int value, char *str, int base);
 

src/arch/x86/syscalls.asm

@@ -0,0 +1,7 @@
+[BITS 64]
+
+section .text
+global _syscall_entrypoint:function (_syscall_entrypoint.end - _syscall_entrypoint)
+_syscall_entrypoint:
+  sysret
+.end:

src/arch/x86/syscalls.cpp

@@ -0,0 +1,40 @@
+//
+// Created by Stepan Usatiuk on 26.10.2023.
+//
+
+#include "syscalls.hpp"
+
+#include <cstdint>
+
+#include "asserts.hpp"
+#include "gdt.hpp"
+#include "misc.hpp"
+
+struct STAR {
+    unsigned ret_cs_ss : 16;
+    unsigned call_cs_ss : 16;
+    unsigned unused : 32;
+} __attribute__((packed));
+
+static_assert(sizeof(STAR) == 8);
+
+void setup_syscalls() {
+    union {
+        STAR star;
+        uint64_t bytes;
+    } newstar{};
+
+    newstar.star.ret_cs_ss = (GDTSEL(gdt_data_user) - 8) | 0x3;
+    assert(newstar.star.ret_cs_ss + 8 == (GDTSEL(gdt_data_user) | 0x3));
+    assert(newstar.star.ret_cs_ss + 16 == (GDTSEL(gdt_code_user) | 0x3));
+
+    newstar.star.call_cs_ss = (GDTSEL(gdt_code));
+    assert(newstar.star.call_cs_ss == GDTSEL(gdt_code));
+    assert(newstar.star.call_cs_ss + 8 == GDTSEL(gdt_data));
+
+    wrmsr(0xc0000081, newstar.bytes);
+    wrmsr(0xc0000082, reinterpret_cast<uint64_t>(&_syscall_entrypoint));
+    wrmsr(0xc0000084, 0);
+
+    wrmsr(0xC0000080, rdmsr(0xC0000080) | 0b1);
+}

src/arch/x86/syscalls.hpp

@@ -0,0 +1,12 @@
+//
+// Created by Stepan Usatiuk on 26.10.2023.
+//
+
+#ifndef OS2_SYSCALLS_HPP
+#define OS2_SYSCALLS_HPP
+
+void setup_syscalls();
+
+extern "C" void _syscall_entrypoint();
+
+#endif//OS2_SYSCALLS_HPP