Update on chapter 3 notes.

notes/chapter3-page-tables

@@ -33,6 +33,8 @@ vocabulary, acronyms, et caetera:
 - Virtual memory = ideas and techniques associated with managing page tables
                    used to achieve isolation & such
 - direct mapping = virt@ == phy@
+- KVM/UVM = Kernel/User virtual memory
+- a page table = 512 PTEs = can be contained into 1 memory page
 
 virt@ = [ 25-bit EXT ; 27-bit index ; 12-bit offset ]
         ↑64          ↑39            ↑12             ↑0
@@ -83,3 +85,32 @@ XV6 kernel page table:
   => these pages are related to the processes (`kstack` in the `proc` structure)
   => the kstack is followed by an invalid guard page (PTE_V not set)
      to prevent memory corruption from stack overflows
+
+Virtual memory functions:
+  - walk: find PTE for a virt@
+  - mappages: install PTEs for new mappings
+  - kvm_* = kernel virtual memory functions (kernel page table)
+  - uvm_* = same but for a user process
+  - copyin = copy data from user space to kernel space
+  - copyout = copy data from kernel space to user space
+  - copyinstr = copy a null-terminated string from user to kernel
+              = used for paths given in syscalls for example
+  - kvminit sets the root kernel page table created by kvmmake
+  - kvmmake creates a direct-map page table for the kernel
+    1. create the root kernel page table with a call to `kalloc`
+       (kalloc provides a pointer to a page, which is the type `pagetable_t`)
+    2. call kvmmap multiple times to set a few direct-map pages
+       kvmmap adds mapping to the kernel page table (when booting only), doesn't flush TLB or enable paging
+       mapped stuff:
+         uart registers, virtio mmio disk interface, PLIC, kernel text and data
+         and trampoline (for trap entry/exit) is mapped to the highest virtual address in the kernel
+    3. proc_mapstacks
+
+Function signatures (for reference):
+  void kvminit(void); // set the kernel root page table
+  pagetable_t kvmmake(void); // create the kernel page table
+  void kvmmap(pagetable_t, uint64 virt@, uint64 phy@, uint64 sz, int perm); // add PTEs to the kernel page table
+    (this is only a call to mappages + a call to "panic" in case of an error)
+  int mappages(pagetable_t, uint64 virt@, uint64 size, uint64 phy@, int perm); // create PTEs
+  pte_t * walk(pagetable_t pagetable, uint64 va, int alloc); // virt@ -> PTE
+  uint64 walkaddr(pagetable_t pagetable, uint64 va); // virt@ -> phy@