[src/trunk]: src/sys/arch/alpha/alpha - Give lwp0 the same kernel thread back...

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

details:   https://anonhg.NetBSD.org/src/rev/3b90136fe998
branches:  trunk
changeset: 944096:3b90136fe998
user:      thorpej <thorpej%NetBSD.org@localhost>
date:      Fri Sep 18 00:11:31 2020 +0000

description:
- Give lwp0 the same kernel thread backstop as all of the others get
  in cpu_lwp_fork().
- Add a bunch of comments to lwp_trampoline() to explain what's going on.
- In alpha_softint_switchto() tweak how we save the RA in the stack frame.

diffstat:

 sys/arch/alpha/alpha/locore.s |  39 +++++++++++++++++++++++++--------------
 1 files changed, 25 insertions(+), 14 deletions(-)

diffs (95 lines):

diff -r b747e3e1c02b -r 3b90136fe998 sys/arch/alpha/alpha/locore.s
--- a/sys/arch/alpha/alpha/locore.s     Fri Sep 18 00:09:29 2020 +0000
+++ b/sys/arch/alpha/alpha/locore.s     Fri Sep 18 00:11:31 2020 +0000
@@ -1,4 +1,4 @@
-/* $NetBSD: locore.s,v 1.134 2020/09/17 00:48:56 thorpej Exp $ */
+/* $NetBSD: locore.s,v 1.135 2020/09/18 00:11:31 thorpej Exp $ */
 
 /*-
  * Copyright (c) 1999, 2000, 2019 The NetBSD Foundation, Inc.
@@ -67,7 +67,7 @@
 
 #include <machine/asm.h>
 
-__KERNEL_RCSID(0, "$NetBSD: locore.s,v 1.134 2020/09/17 00:48:56 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: locore.s,v 1.135 2020/09/18 00:11:31 thorpej Exp $");
 
 #include "assym.h"
 
@@ -159,8 +159,17 @@
 
        /*
         * All ready to go!  Call main()!
+        *
+        * We're going to play a little trick there, though.  We are
+        * going to fake our return address as the kthread backstop.
+        * Hitting the backstop will trigger a panic, and we want lwp0
+        * to work like other kthreads in that regard.  We will still
+        * leep the "main returned" backstop here in case something
+        * goes horribly wrong.
         */
-       CALL(main)
+       lda     ra, alpha_kthread_backstop
+       jsr     s0, main
+       ldgp    gp, 0(s0)
 
        /* This should never happen. */
        PANIC("main() returned",Lmain_returned_pmsg)
@@ -669,7 +678,7 @@
        ldq     a3, L_PCB(a0)                   /* a3 = from->l_pcb */
 
        lda     sp, -16(sp)                     /* set up stack frame */
-       stq     ra, (16-8)(sp)                  /* save ra */
+       stq     ra, 0(sp)                       /* save ra */
 
        /*
         * Step 1: Save the current LWP's context.  We don't
@@ -727,7 +736,7 @@
        SET_CURLWP(s0)                  /* clobbers a0, v0, t0, t8..t11 */
        ldq     sp, PCB_HWPCB_KSP(a3)           /* restore sp */
        ldq     s0, PCB_CONTEXT+(0 * 8)(a3)     /* restore s0 */
-       ldq     ra, (16-8)(sp)                  /* restore ra */
+       ldq     ra, 0(sp)                       /* restore ra */
        lda     sp, 16(sp)                      /* pop stack frame */
        RET
        END(alpha_softint_switchto)
@@ -746,7 +755,7 @@
         * Step 2: Pop alpha_softint_switchto()'s stack frame
         * and return.
         */
-       ldq     ra, (16-8)(sp)                  /* restore ra */
+       ldq     ra, 0(sp)                       /* restore ra */
        lda     sp, 16(sp)                      /* pop stack frame */
        RET
        END(alpha_softint_return)
@@ -851,20 +860,22 @@
 /*
  * lwp_trampoline()
  *
- * Arrange for a function to be invoked neatly, after a cpu_lwp_fork().
+ * Arrange for a function to be invoked neatly, after a cpu_lwp_fork(),
+ * which has set up our pcb_context for us.  But we actually *get here*
+ * via cpu_switchto(), which returns the LWP we switched away from in v0.
  *
  * Invokes the function specified by the s0 register with the return
  * address specified by the s1 register and with one argument specified
  * by the s2 register.
  */
 LEAF_NOPROFILE(lwp_trampoline, 0)
-       mov     v0, a0
-       mov     s3, a1
-       CALL(lwp_startup)
-       mov     s0, pv
-       mov     s1, ra
-       mov     s2, a0
-       jmp     zero, (pv)
+       mov     v0, a0          /* a0 = prev_lwp (from cpu_switchto()) */
+       mov     s3, a1          /* a1 = new_lwp (that's us!) */
+       CALL(lwp_startup)       /* lwp_startup(prev_lwp, new_lwp); */
+       mov     s0, pv          /* pv = func */
+       mov     s1, ra          /* ra = (probably exception_return()) */
+       mov     s2, a0          /* a0 = arg */
+       jmp     zero, (pv)      /* func(arg) */
        END(lwp_trampoline)
 
 /**************************************************************************/