Index: amd64/amd64/exception.S
===================================================================
--- amd64/amd64/exception.S	(revision 221079)
+++ amd64/amd64/exception.S	(working copy)
@@ -339,6 +339,9 @@
  * and the new privilige level.  We are still running on the old user stack
  * pointer.  We have to juggle a few things around to find our stack etc.
  * swapgs gives us access to our PCPU space only.
+ *
+ * We do not support invoking this from a custom %cs or %ss (e.g. using
+ * entries from an LDT).
  */
 IDTVEC(fast_syscall)
 	swapgs
@@ -380,7 +383,37 @@
 	movq	%rsp,%rdi
 	call	syscall
 	movq	PCPU(CURPCB),%rax
+	testq	$PCB_FULL_IRET,PCB_FLAGS(%rax)
+	jne	3f
+1:	/* Check for and handle AST's on return to userland. */
+	cli
+	movq	PCPU(CURTHREAD),%rax
+	testl	$TDF_ASTPENDING | TDF_NEEDRESCHED,TD_FLAGS(%rax)
+	je	2f
+	sti
+	movq	%rsp, %rdi
+	call	ast
+	jmp	1b
+2:	/* Restore preserved registers. */
 	MEXITCOUNT
+	movq	TF_RDI(%rsp),%rdi	/* bonus; preserve arg 1 */
+	movq	TF_RSI(%rsp),%rsi	/* bonus: preserve arg 2 */
+	movq	TF_RDX(%rsp),%rdx	/* return value 2 */
+	movq	TF_RAX(%rsp),%rax	/* return value 1 */
+	movq	TF_RBX(%rsp),%rbx	/* C preserved */
+	movq	TF_RBP(%rsp),%rbp	/* C preserved */
+	movq	TF_R12(%rsp),%r12	/* C preserved */
+	movq	TF_R13(%rsp),%r13	/* C preserved */
+	movq	TF_R14(%rsp),%r14	/* C preserved */
+	movq	TF_R15(%rsp),%r15	/* C preserved */
+	movq	TF_RFLAGS(%rsp),%r11	/* original %rflags */
+	movq	TF_RIP(%rsp),%rcx	/* original %rip */
+	movq	TF_RSP(%rsp),%r9	/* user stack pointer */
+	movq	%r9,%rsp		/* original %rsp */
+	swapgs
+	sysretq
+3:	/* Requested full context restore, use doreti for that. */
+	MEXITCOUNT
 	jmp	doreti
 
 /*