x86: Store pointer to the current thread in gs:0

Apparently, reading from dr3 is slower than reading from memory
with cache hit.

Also, depending on hypervisor configuration, accessing dr3 may cause
a VM exit (and, at least on kvm, it does), what makes it much slower
than a memory access even when there is a cache miss.

headers/private/kernel/arch/x86/32/descriptors.h

@@ -38,6 +38,8 @@
 #define USER_CODE_SELECTOR	((USER_CODE_SEGMENT << 3) | DPL_USER)
 #define USER_DATA_SELECTOR	((USER_DATA_SEGMENT << 3) | DPL_USER)
 
+#define KERNEL_TLS_SELECTOR	((KERNEL_TLS_SEGMENT << 3) | DPL_KERNEL)
+
 
 #ifndef _ASSEMBLER
 	// this file can also be included from assembler as well

headers/private/kernel/arch/x86/arch_thread.h

@@ -30,9 +30,6 @@ void x86_restart_syscall(struct iframe* frame);
 void x86_set_tls_context(Thread* thread);
 
 
-#ifdef __x86_64__
-
-
 static inline Thread*
 arch_thread_get_current_thread(void)
 {
@@ -42,6 +39,9 @@ arch_thread_get_current_thread(void)
 }
 
 
+#ifdef __x86_64__
+
+
 static inline void
 arch_thread_set_current_thread(Thread* t)
 {
@@ -59,18 +59,10 @@ arch_thread_set_current_thread(Thread* t)
 void arch_syscall_64_bit_return_value(void);
 
 
-static inline Thread*
-arch_thread_get_current_thread(void)
-{
-	Thread* t = (Thread*)x86_read_dr3();
-	return t;
-}
-
-
 static inline void
 arch_thread_set_current_thread(Thread* t)
 {
-	x86_write_dr3(t);
+	asm volatile("mov %0, %%gs:0" : : "r" (t));
 }
 
 
@@ -82,3 +74,4 @@ arch_thread_set_current_thread(Thread* t)
 #endif
 
 #endif /* _KERNEL_ARCH_x86_THREAD_H */
+