fix cs register after sysretq

1 files changed, 6 insertions, 10 deletions

diff --git a/arch/x86_64/src/context_switching/main.cpp b/arch/x86_64/src/context_switching/main.cpp
index 2a2a188..8d1c019 100644
--- a/arch/x86_64/src/context_switching/main.cpp
+++ b/arch/x86_64/src/context_switching/main.cpp
@@ -42,7 +42,7 @@ namespace teachos::arch::context_switching
                    : "memory");
       asm volatile("syscall");
 
-      video::vga::text::write("Successfully made a SYSCALL and returned back with SYSRETQ!",
+      video::vga::text::write("Successfully made a SYSCALL and returned with SYSRETQ!",
                               video::vga::text::common_attributes::green_on_black);
     }
 
@@ -70,16 +70,12 @@ namespace teachos::arch::context_switching
     {
       uint64_t const syscall_function = reinterpret_cast<uint64_t>(syscall_handler);
       kernel::cpu::write_msr(IA32_LSTAR_ADDRESS, syscall_function);
-      kernel::cpu::write_msr(IA32_FMASK_ADDRESS, 1 << 9U);  // Disable interrupt flag during syscall
-
-      // @MTO: This produces following values:
-      // After SYSCALL: CS = 0x10, SS = 0x18
-      // After SYSRETQ: CS = 0x43, SS = 0x3b
-      //
-      // We probably need to modify our GDT, because the cs+8 = ss is an issue we cannot solve.
-      // Also, CS = 0x43 is weird. I expected it to be 0x33.
+      kernel::cpu::write_msr(IA32_FMASK_ADDRESS, 1 << 9U);  // Disable interrupt flag during syscall.
+
       uint64_t kernel_cs = KERNEL_CODE_SEGMENT_SELECTOR;
-      uint64_t user_cs = USER_CODE_SEGMENT_SELECTOR;
+      // We want to provide the user code segment, but the instruction calculates + 0x10 to fill the
+      // cs register (See https://www.felixcloutier.com/x86/sysret).
+      uint64_t user_cs = USER_CODE_SEGMENT_SELECTOR - 0x10;
       uint64_t star_value = (kernel_cs << 32) | (user_cs << 48);
       kernel::cpu::write_msr(IA32_STAR_ADDRESS, star_value);