Bare metal: tweaks pertaining to physical memory handling

- Factor __invert_memory() into two routines, to make it
  easier for other libc routines to create their own negative
  address mappings
- Tweak __invert_memory() to map only the usable memory as
  given in the 0xe820 memory map, and no more
- Arrange for VGA console initialization to map video memory
  separately into virtual memory (in case needed)
- Rename libc/vga/vga-init.S to libc/vga/rlinit-vga.S, to
  reduce confusion as to the module's purpose

libc/intrin/mman.greg.c

@@ -112,19 +112,34 @@ static noasan textreal void __normalize_e820(struct mman *mm) {
   mm->e820n = n;
 }
 
+/**
+ * Identity maps an area of physical memory to its negative address.
+ */
+noasan textreal void __invert_memory_area(struct mman *mm, uint64_t *pml4t,
+                                          uint64_t ps, uint64_t size,
+                                          uint64_t pte_flags) {
+  uint64_t pe = ps + size, p, *m;
+  ps = ROUNDDOWN(ps, 4096);
+  pe = ROUNDUP(pe, 4096);
+  for (p = ps; p != pe; p += 4096) {
+    m = __get_virtual(mm, pml4t, BANE + p, true);
+    if (m && !(*m & PAGE_V)) {
+      *m = p | PAGE_V | pte_flags;
+    }
+  }
+}
+
 /**
  * Identity maps all usable physical memory to its negative address.
  */
 static noasan textreal void __invert_memory(struct mman *mm, uint64_t *pml4t) {
   uint64_t i, j, *m, p, pe;
   for (i = 0; i < mm->e820n; ++i) {
-    for (p = mm->e820[i].addr, pe = mm->e820[i].addr + mm->e820[i].size;
-         p != pe + 0x200000; p += 4096) {
-      m = __get_virtual(mm, pml4t, BANE + p, true);
-      if (m && !(*m & PAGE_V)) {
-        *m = p | PAGE_V | PAGE_RW;
-      }
-    }
+    uint64_t ps = mm->e820[i].addr, size = mm->e820[i].size;
+    /* ape/ape.S has already mapped the first 2 MiB of physical memory. */
+    if (ps < 0x200000 && ps + size <= 0x200000)
+      continue;
+    __invert_memory_area(mm, pml4t, ps, size, PAGE_RW);
   }
 }
 

libc/runtime/pc.internal.h

@@ -185,6 +185,8 @@ struct IdtDescriptor {
 uint64_t *__get_virtual(struct mman *, uint64_t *, int64_t, bool);
 uint64_t __clear_page(uint64_t);
 uint64_t __new_page(struct mman *);
+void __invert_memory_area(struct mman *, uint64_t *, uint64_t, uint64_t,
+                          uint64_t);
 void __map_phdrs(struct mman *, uint64_t *, uint64_t);
 
 forceinline unsigned char inb(unsigned short port) {

libc/vga/writev-vga.c

@@ -60,6 +60,7 @@ ssize_t sys_writev_vga(struct Fd *fd, const struct iovec *iov, int iovlen) {
 
 __attribute__((__constructor__)) static textstartup void _vga_init(void) {
   if (IsMetal()) {
+    struct mman *mm = (struct mman *)(BANE + 0x0500);
     void * const vid_buf = (void *)(BANE + 0xb8000ull);
     /*
      * Get the initial cursor position from the BIOS data area.  Also get
@@ -74,6 +75,9 @@ __attribute__((__constructor__)) static textstartup void _vga_init(void) {
             chr_ht_hi = *(uint8_t *)(BANE + 0x0486ull);
     if (chr_ht_hi != 0 || chr_ht > 32)
       chr_ht = 32;
+    /* Make sure the video buffer is mapped into virtual memory. */
+    __invert_memory_area(mm, __get_pml4t(), (uint64_t)vid_buf,
+                         2 * VGA_TTY_HEIGHT * VGA_TTY_WIDTH, PAGE_RW);
     /*
      * Initialize our tty structure from the current screen contents,
      * current cursor position, & character height.