[src/trunk]: src/sys/arch/ia64/ia64 Fix some comments and add new comment exp...

[scole <scole%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/34fc96a1cbe5
branches:  trunk
changeset: 349864:34fc96a1cbe5
user:      scole <scole%NetBSD.org@localhost>
date:      Mon Dec 26 19:46:59 2016 +0000

description:
Fix some comments and add new comment explaining probable bug in cpu_switchto().  Not sure of a fix for it yet

diffstat:

 sys/arch/ia64/ia64/context.S |  26 ++++++++++++++++++++++----
 1 files changed, 22 insertions(+), 4 deletions(-)

diffs (54 lines):

diff -r 0678589315f1 -r 34fc96a1cbe5 sys/arch/ia64/ia64/context.S
--- a/sys/arch/ia64/ia64/context.S      Mon Dec 26 18:47:00 2016 +0000
+++ b/sys/arch/ia64/ia64/context.S      Mon Dec 26 19:46:59 2016 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: context.S,v 1.6 2016/08/05 16:46:56 scole Exp $        */
+/*     $NetBSD: context.S,v 1.7 2016/12/26 19:46:59 scole Exp $        */
 
 /*
  * Copyright (c) 2003 Marcel Moolenaar
@@ -821,7 +821,7 @@
         * Save old context, unless the LWP is exiting.
         */
        cmp.eq          p6,p0=r0,in0            // p6 = (in0 == 0)
-       add             r14=PC_CURLWP,r13       // r2 = &ci->ci_curlwp
+       add             r14=PC_CURLWP,r13       // r14 = &ci->ci_curlwp
        ;;
 }
 {      .mii
@@ -840,7 +840,7 @@
        nop             0
        /*
         * Switch to new context, if p6 == true.
-        * We assum to return to restorectx_return_here for swapped context.
+        * We assume to return to restorectx_return_here for swapped context.
         */
 (p6)   ld8             out0=[r3]               // out0 = pcb of &newlwp
 (p6)   br.call.sptk.many rp=restorectx         // if (p6) restorectx(out0)
@@ -855,7 +855,25 @@
        br.call.sptk.many rp=swapctx
        ;;
 }
-
+       /*
+        * XXX seems to be a bug here...
+        *
+        * swapctx() and restorectx() (which is called by swapctx())
+        * both adjust the bspstore (and hence bsp) registers.  When
+        * returning to "restorectx_return_here:" the bspstore can
+        * be greater than bsp, a state known as an "Incomplete Register Frame".
+        *
+        * That is not necessarily fatal in itself, but apparently the RSE
+        * engine sets bsp=bspstore to adjust, which re-syncs the stacked
+        * registers (r32-r127).  This clobbers our local registers below
+        * (loc0 and loc1) on the br return from swapctx(), and you get all
+        * kind of mysterious exceptions depeding on what gets restored from
+        * the backing store.
+        *
+        * Not sure of a proper fix is yet or how cpu_switchto should/can
+        * be interacting with restorectx()/swapctx()
+        * 
+        */
 restorectx_return_here:
 {      .mib
        mov             r8=r9                   // r8(ret0) = oldlwp