[src/trunk]: src/sys/arch/amd64/stand/prekern Explain more.

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

details:   https://anonhg.NetBSD.org/src/rev/679f4a891fde
branches:  trunk
changeset: 971882:679f4a891fde
user:      maxv <maxv%NetBSD.org@localhost>
date:      Thu May 07 17:10:02 2020 +0000

description:
Explain more.

diffstat:

 sys/arch/amd64/stand/prekern/mm.c |  40 ++++++++++++++++++++++++--------------
 1 files changed, 25 insertions(+), 15 deletions(-)

diffs (87 lines):

diff -r cb6c292c8f74 -r 679f4a891fde sys/arch/amd64/stand/prekern/mm.c
--- a/sys/arch/amd64/stand/prekern/mm.c Thu May 07 16:49:59 2020 +0000
+++ b/sys/arch/amd64/stand/prekern/mm.c Thu May 07 17:10:02 2020 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: mm.c,v 1.25 2020/02/15 10:41:25 maxv Exp $     */
+/*     $NetBSD: mm.c,v 1.26 2020/05/07 17:10:02 maxv Exp $     */
 
 /*
  * Copyright (c) 2017-2020 The NetBSD Foundation, Inc. All rights reserved.
@@ -248,7 +248,7 @@
 }
 
 static paddr_t
-bootspace_getend(void)
+bootspace_get_kern_segs_end_pa(void)
 {
        paddr_t pa, max = 0;
        size_t i;
@@ -410,21 +410,29 @@
         * the number of pages entered is lower.
         */
 
-       /* Create the page tree */
+       /* Create the page tree, starting at a random VA */
        size = (NKL2_KIMG_ENTRIES + 1) * NBPD_L2;
        randva = mm_randva_kregion(size, PAGE_SIZE);
 
-       /* Enter the area and build the ELF info */
-       bootpa = bootspace_getend();
+       /* The "boot" region begins right after the kernel segments */
+       bootpa = bootspace_get_kern_segs_end_pa();
+
+       /* The prekern consumed some memory up until pa_avail, this covers
+        * SYM+REL and EXTRA */
        size = (pa_avail - bootpa);
        npages = size / PAGE_SIZE;
+
+       /* Enter the whole area linearly */
        for (i = 0; i < npages; i++) {
                mm_enter_pa(bootpa + i * PAGE_SIZE,
                    randva + i * PAGE_SIZE, MM_PROT_READ|MM_PROT_WRITE);
        }
+
+       /* At this point both "head" and "boot" are mapped, so we can build
+        * the ELF info */
        elf_build_boot(randva, bootpa);
 
-       /* Enter the ISA I/O MEM */
+       /* Map the ISA I/O MEM right after EXTRA */
        iom_base = randva + npages * PAGE_SIZE;
        npages = IOM_SIZE / PAGE_SIZE;
        for (i = 0; i < npages; i++) {
@@ -451,23 +459,25 @@
  * +------------+-----------------+---------------+------------------+-------+
  * | ELF HEADER | SECTION HEADERS | KERN SECTIONS | SYM+REL SECTIONS | EXTRA |
  * +------------+-----------------+---------------+------------------+-------+
- * Which we abstract into several "regions":
+ * This was done in the loadfile_elf32.c:loadfile_dynamic() function.
+ *
+ * We abstract this layout into several "regions":
  * +------------------------------+---------------+--------------------------+
- * |         Head region          | Several segs  |       Boot region        |
+ * |         Head region          |  Kernel segs  |       Boot region        |
  * +------------------------------+---------------+--------------------------+
- * See loadfile_elf32.c:loadfile_dynamic() for the details.
  *
  * There is a variable number of independent regions we create: one head,
  * several kernel segments, one boot. They are all mapped at random VAs.
  *
- * Head contains the ELF Header and ELF Section Headers, and we use them to
- * map the rest of the regions. Head must be placed in both virtual memory
- * and physical memory *before* the rest.
+ * "Head" contains the ELF Header and ELF Section Headers, and we use them to
+ * map the rest of the regions. Head must be placed *before* the other
+ * regions, in both virtual memory and physical memory.
  *
- * The Kernel Sections are mapped at random VAs using individual segments
- * in bootspace.
+ * The "Kernel Segments" contain the kernel SHT_NOBITS and SHT_PROGBITS
+ * sections, in a 1:1 manner (one segment is associated with one section).
+ * The segments are mapped at random VAs and referenced in bootspace.segs[].
  *
- * Boot contains various information, including the ELF Sym+Rel sections,
+ * "Boot" contains various information, including the ELF Sym+Rel sections,
  * plus extra memory the prekern has used so far; it is a region that the
  * kernel will eventually use for module_map. Boot is placed *after* the
  * other regions in physical memory. In virtual memory however there is no