The following reply was made to PR amd64/151167; it has been noted by GNATS.

From: (dfilter service)
Subject: Re: amd64/151167: commit references a PR
Date: Sun,  3 Oct 2010 13:52:22 +0000 (UTC)

 Author: kib
 Date: Sun Oct  3 13:52:17 2010
 New Revision: 213382
   The makectx() function, used by kdb_trap() to reconstruct pcb from
   trap frame when trap initiated kdb entry, incorrectly calculated the
   value of %rsp for trapped thread.
   According to Intel(R) 64 and IA-32 Architectures Software Developer's Manual
   Volume 3A: System Programming Guide, Part 1, rev. 035, 6.14.2 64-Bit Mode
   Stack Frame, "64-bit mode ... pushes SS:RSP unconditionally, rather than
   only on a CPL change."
   Even assuming the conditional push of the %ss:%rsp, the calculation
   was still wrong because sizeof(tf_ss) + sizeof(tf_rsp) == 16 on amd64.
   Always use the tf_rsp from trap frame. The change supposedly fixes
   stepping when using kgdb backend for kdb.
   Submitted by:        Zhouyi Zhou <zhouzhouyi gmail com>
   PR:  amd64/151167
   Reviewed by: avg
   MFC after:   1 week
 Modified: head/sys/amd64/amd64/machdep.c
 --- head/sys/amd64/amd64/machdep.c     Sun Oct  3 13:13:10 2010        
 +++ head/sys/amd64/amd64/machdep.c     Sun Oct  3 13:52:17 2010        
 @@ -1799,7 +1799,7 @@ makectx(struct trapframe *tf, struct pcb
        pcb->pcb_rbp = tf->tf_rbp;
        pcb->pcb_rbx = tf->tf_rbx;
        pcb->pcb_rip = tf->tf_rip;
 -      pcb->pcb_rsp = (ISPL(tf->tf_cs)) ? tf->tf_rsp : (long)(tf + 1) - 8;
 +      pcb->pcb_rsp = tf->tf_rsp;
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