[kernel] Add a request IOPL syscall

Using the new ability to modify user rflags, add a syscall for a process
to request its IOPL be changed.

definitions/objects/system.def

@@ -26,4 +26,10 @@ object system : kobject {
         param size size             # Size of the area, in pages
         param flags uint32          # Flags to apply to the created VMA
     }
+
+    # Request the kernel change the IOPL for this process. The only values
+    # that make sense are 0 and 3.
+    method request_iopl {
+        param iopl uint             # The IOPL to set for this process
+    }
 }

src/kernel/syscalls/system.cpp

@@ -1,6 +1,7 @@
 #include "j6/errors.h"
 #include "j6/types.h"
 
+#include "cpu.h"
 #include "device_manager.h"
 #include "frame_allocator.h"
 #include "log.h"
@@ -77,4 +78,17 @@ system_map_phys(j6_handle_t handle, j6_handle_t * area, uintptr_t phys, size_t s
     return j6_status_ok;
 }
 
+j6_status_t
+system_request_iopl(j6_handle_t handle, unsigned iopl)
+{
+    // TODO: check capabilities on sys handle
+    if (iopl != 0 && iopl != 3)
+        return j6_err_invalid_arg;
+
+    constexpr uint64_t mask = 3 << 12;
+    cpu_data &cpu = current_cpu();
+    cpu.rflags3 = (cpu.rflags3 & ~mask) | ((iopl << 12) & mask);
+    return j6_status_ok;
+}
+
 } // namespace syscalls