[src/trunk]: src/sys/arch/amd64/amd64 Explain a little what we are doing. Als...

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

details:   https://anonhg.NetBSD.org/src/rev/7f17076808ef
branches:  trunk
changeset: 347330:7f17076808ef
user:      maxv <maxv%NetBSD.org@localhost>
date:      Sun Aug 21 10:07:15 2016 +0000

description:
Explain a little what we are doing. Also, make sure gdt_init_cpu is called
on the currently running CPU. Theoretically, we could put the same KASSERT
in gdt_reload_cpu, but the associated IPI is never sent, which is another
issue.

diffstat:

 sys/arch/amd64/amd64/gdt.c |  42 +++++++++++++++++++++++++++++++-----------
 1 files changed, 31 insertions(+), 11 deletions(-)

diffs (132 lines):

diff -r 4a98633e263b -r 7f17076808ef sys/arch/amd64/amd64/gdt.c
--- a/sys/arch/amd64/amd64/gdt.c        Sun Aug 21 09:53:25 2016 +0000
+++ b/sys/arch/amd64/amd64/gdt.c        Sun Aug 21 10:07:15 2016 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: gdt.c,v 1.31 2016/08/21 08:30:22 christos Exp $        */
+/*     $NetBSD: gdt.c,v 1.32 2016/08/21 10:07:15 maxv Exp $    */
 
 /*-
  * Copyright (c) 1996, 1997, 2009 The NetBSD Foundation, Inc.
@@ -37,7 +37,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: gdt.c,v 1.31 2016/08/21 08:30:22 christos Exp $");
+__KERNEL_RCSID(0, "$NetBSD: gdt.c,v 1.32 2016/08/21 10:07:15 maxv Exp $");
 
 #include "opt_multiprocessor.h"
 #include "opt_xen.h"
@@ -105,7 +105,8 @@
 }
 
 /*
- * Initialize the GDT.
+ * Initialize the GDT. We already have a gdtstore, which was temporarily used
+ * by the bootstrap code. Now, we allocate a new gdtstore, and put it in cpu0.
  */
 void
 gdt_init(void)
@@ -120,11 +121,18 @@
        gdt_next = 0;
        gdt_free = GNULL_SEL;
        gdt_dynavail =
-           (gdt_size - DYNSEL_START) / sizeof (struct sys_segment_descriptor);
+           (gdt_size - DYNSEL_START) / sizeof(struct sys_segment_descriptor);
 
        old_gdt = gdtstore;
+
+       /* Allocate MAXGDTSIZ bytes of virtual memory. */
        gdtstore = (char *)uvm_km_alloc(kernel_map, MAXGDTSIZ, 0,
            UVM_KMF_VAONLY);
+
+       /*
+        * Allocate only MINGDTSIZ bytes of physical memory. We will grow this
+        * area in gdt_grow at run-time if needed.
+        */
        for (va = (vaddr_t)gdtstore; va < (vaddr_t)gdtstore + MINGDTSIZ;
            va += PAGE_SIZE) {
                pg = uvm_pagealloc(NULL, 0, NULL, UVM_PGA_ZERO);
@@ -135,17 +143,21 @@
                    VM_PROT_READ | VM_PROT_WRITE, 0);
        }
        pmap_update(pmap_kernel());
+
+       /* Copy the initial bootstrap GDT into the new area. */
        memcpy(gdtstore, old_gdt, DYNSEL_START);
        ci->ci_gdt = (void *)gdtstore;
 #ifndef XEN
        set_sys_segment(GDT_ADDR_SYS(gdtstore, GLDT_SEL), ldtstore,
            LDT_SIZE - 1, SDT_SYSLDT, SEL_KPL, 0);
 #endif
+
        gdt_init_cpu(ci);
 }
 
 /*
- * Allocate shadow GDT for a slave CPU.
+ * Allocate shadow GDT for a secondary CPU. It contains the same values as the
+ * GDT present in cpu0 (gdtstore).
  */
 void
 gdt_alloc_cpu(struct cpu_info *ci)
@@ -157,6 +169,7 @@
 
        ci->ci_gdt = (union descriptor *)uvm_km_alloc(kernel_map, max_len,
            0, UVM_KMF_VAONLY);
+
        for (va = (vaddr_t)ci->ci_gdt; va < (vaddr_t)ci->ci_gdt + min_len;
            va += PAGE_SIZE) {
                while ((pg = uvm_pagealloc(NULL, 0, NULL, UVM_PGA_ZERO))
@@ -167,20 +180,22 @@
                    VM_PROT_READ | VM_PROT_WRITE, 0);
        }
        pmap_update(pmap_kernel());
+
        memset(ci->ci_gdt, 0, min_len);
        memcpy(ci->ci_gdt, gdtstore, gdt_size);
 }
 
-
 /*
- * Load appropriate gdt descriptor; we better be running on *ci
- * (for the most part, this is how a CPU knows who it is).
+ * Load appropriate GDT descriptor into the currently running CPU, which must
+ * be ci.
  */
 void
 gdt_init_cpu(struct cpu_info *ci)
 {
        struct region_descriptor region;
 
+       KASSERT(curcpu() == ci);
+
 #ifndef XEN
        setregion(&region, ci->ci_gdt, (uint16_t)(MAXGDTSIZ - 1));
 #else
@@ -206,7 +221,9 @@
 
 #if !defined(XEN) || defined(USER_LDT)
 /*
- * Grow the GDT.
+ * Grow the GDT. The GDT is present on each CPU, so we need to iterate over all
+ * of them. We already have the virtual memory, we only need to grow the
+ * physical memory.
  */
 static void
 gdt_grow(void)
@@ -305,10 +322,13 @@
        int slot;
 
        mutex_enter(&cpu_lock);
+
        slot = gdt_get_slot();
-       set_sys_gdt(slot, tss, sizeof (struct x86_64_tss) - 1,
-           SDT_SYS386TSS, SEL_KPL, 0);
+       set_sys_gdt(slot, tss, sizeof(struct x86_64_tss) - 1, SDT_SYS386TSS,
+           SEL_KPL, 0);
+
        mutex_exit(&cpu_lock);
+
        return GDYNSEL(slot, SEL_KPL);
 #else  /* XEN */
        /* TSS, what for? */