Give a %fs and %gs to userland. Use swapgs to obtain the kernel %GS.base

value on entry and exit.  This isn't as easy as it sounds because when
we recursively trap or interrupt, we have to avoid duplicating the
swapgs instruction or we end up back with the userland %gs.  I implemented
this by testing TF_CS to see if we're coming from supervisor mode
already, and check for returning to supervisor. To avoid a race with
interrupts in the brief period after beginning executing the handler and
before the swapgs, convert all trap gates to interrupt gates, and reenable
interrupts immediately after the swapgs.  I am not happy with this.
There are other possible ways to do this that should be investigated.
(eg: storing the GS.base MSR value in the trapframe)

Add some sysarch functions to let the userland code get to this.

Approved by:	re (blanket amd64/*)

1 files changed, 19 insertions, 1 deletions

diff --git a/sys/amd64/include/sysarch.h b/sys/amd64/include/sysarch.h
index c33f7b8..f14ee31 100644
--- a/sys/amd64/include/sysarch.h
+++ b/sys/amd64/include/sysarch.h
@@ -34,9 +34,27 @@
  */
 
 /*
- * Architecture specific syscalls (i386)
+ * Architecture specific syscalls (AMD64)
  */
 #ifndef _MACHINE_SYSARCH_H_
 #define _MACHINE_SYSARCH_H_
 
+#define	AMD64_GET_FSBASE	0
+#define	AMD64_SET_FSBASE	1
+#define	AMD64_GET_GSBASE	2
+#define	AMD64_SET_GSBASE	3
+
+#if 0 /* these wrappers need to be implemented in libc first */
+#ifndef _KERNEL
+#include <sys/cdefs.h>
+
+__BEGIN_DECLS
+unsigned long amd64_get_fsbase(void);
+unsigned long amd64_set_fsbase(unsigned long);
+unsigned long amd64_get_gsbase(void);
+unsigned long amd64_set_gsbase(unsigned long);
+__END_DECLS
+#endif
+#endif
+
 #endif /* !_MACHINE_SYSARCH_H_ */