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1/* See COPYRIGHT for copyright information. */
2
3#include <inc/x86.h>
4#include <inc/mmu.h>
5#include <inc/error.h>
6#include <inc/string.h>
7#include <inc/assert.h>
8
9#include <kern/pmap.h>
10#include <kern/kclock.h>
11
12// These variables are set by i386_detect_memory()
13static physaddr_t maxpa;	// Maximum physical address
14size_t npage;			// Amount of physical memory (in pages)
15static size_t basemem;		// Amount of base memory (in bytes)
16static size_t extmem;		// Amount of extended memory (in bytes)
17
18// These variables are set in i386_vm_init()
19pde_t* boot_pgdir;		// Virtual address of boot time page directory
20physaddr_t boot_cr3;		// Physical address of boot time page directory
21static char* boot_freemem;	// Pointer to next byte of free mem
22
23struct Page* pages;		// Virtual address of physical page array
24static struct Page_list page_free_list;	// Free list of physical pages
25
26// Global descriptor table.
27//
28// The kernel and user segments are identical (except for the DPL).
29// To load the SS register, the CPL must equal the DPL.  Thus,
30// we must duplicate the segments for the user and the kernel.
31//
32struct Segdesc gdt[] =
33{
34	// 0x0 - unused (always faults -- for trapping NULL far pointers)
35	SEG_NULL,
36
37	// 0x8 - kernel code segment
38	[GD_KT >> 3] = SEG(STA_X | STA_R, 0x0, 0xffffffff, 0),
39
40	// 0x10 - kernel data segment
41	[GD_KD >> 3] = SEG(STA_W, 0x0, 0xffffffff, 0),
42
43	// 0x18 - user code segment
44	[GD_UT >> 3] = SEG(STA_X | STA_R, 0x0, 0xffffffff, 3),
45
46	// 0x20 - user data segment
47	[GD_UD >> 3] = SEG(STA_W, 0x0, 0xffffffff, 3),
48
49	// 0x28 - tss, initialized in idt_init()
50	[GD_TSS >> 3] = SEG_NULL
51};
52
53struct Pseudodesc gdt_pd = {
54	sizeof(gdt) - 1, (unsigned long) gdt
55};
56
57	static int
58nvram_read(int r)
59{
60	return mc146818_read(r) | (mc146818_read(r + 1) << 8);
61}
62
63	void
64i386_detect_memory(void)
65{
66	// CMOS tells us how many kilobytes there are
67	basemem = ROUNDDOWN(nvram_read(NVRAM_BASELO)*1024, PGSIZE);
68	extmem = ROUNDDOWN(nvram_read(NVRAM_EXTLO)*1024, PGSIZE);
69
70	// Calculate the maximum physical address based on whether
71	// or not there is any extended memory.  See comment in <inc/mmu.h>.
72	if (extmem)
73		maxpa = EXTPHYSMEM + extmem;
74	else
75		maxpa = basemem;
76
77	npage = maxpa / PGSIZE;
78
79	cprintf("Physical memory: %dK available, ", (int)(maxpa/1024));
80	cprintf("base = %dK, extended = %dK\n", (int)(basemem/1024), (int)(extmem/1024));
81}
82
83// --------------------------------------------------------------
84// Set up initial memory mappings and turn on MMU.
85// --------------------------------------------------------------
86
87static void check_boot_pgdir(void);
88static void check_page_alloc();
89static void page_check(void);
90static void boot_map_segment(pde_t *pgdir, uintptr_t la, size_t size, physaddr_t pa, int perm);
91
92//
93// A simple physical memory allocator, used only a few times
94// in the process of setting up the virtual memory system.
95// page_alloc() is the real allocator.
96//
97// Allocate n bytes of physical memory aligned on an 
98// align-byte boundary.  Align must be a power of two.
99// Return kernel virtual address.  Returned memory is uninitialized.
100//
101// If we're out of memory, boot_alloc should panic.
102// This function may ONLY be used during initialization,
103// before the page_free_list has been set up.
104// 
105	static void*
106boot_alloc(uint32_t n, uint32_t align)
107{
108	extern char end[];
109	void *v;
110
111	// Initialize boot_freemem if this is the first time.
112	// 'end' is a magic symbol automatically generated by the linker,
113	// which points to the end of the kernel's bss segment -
114	// i.e., the first virtual address that the linker
115	// did _not_ assign to any kernel code or global variables.
116	if (boot_freemem == 0)
117		boot_freemem = end;
118
119	// LAB 2: Your code here:
120	//	Step 1: round boot_freemem up to be aligned properly
121	//		(hint: look in types.h for some handy macros)
122	//	Step 2: save current value of boot_freemem as allocated chunk
123	//	Step 3: increase boot_freemem to record allocation
124	//	Step 4: return allocated chunk
125
126	boot_freemem = (char*) ROUNDUP(boot_freemem,align);
127	v = boot_freemem;
128	boot_freemem = boot_freemem + n; 
129	return v;
130}
131
132// Set up a two-level page table:
133//    boot_pgdir is its linear (virtual) address of the root
134//    boot_cr3 is the physical adresss of the root
135// Then turn on paging.  Then effectively turn off segmentation.
136// (i.e., the segment base addrs are set to zero).
137// 
138// This function only sets up the kernel part of the address space
139// (ie. addresses >= UTOP).  The user part of the address space
140// will be setup later.
141//
142// From UTOP to ULIM, the user is allowed to read but not write.
143// Above ULIM the user cannot read (or write). 
144	void
145i386_vm_init(void)
146{
147	pde_t* pgdir;
148	uint32_t cr0;
149	size_t n;
150
151	// Delete this line:
152	//	panic("i386_vm_init: This function is not finished\n");
153
154	//////////////////////////////////////////////////////////////////////
155	// create initial page directory.
156	pgdir = boot_alloc(PGSIZE, PGSIZE);
157	memset(pgdir, 0, PGSIZE);
158	boot_pgdir = pgdir;
159	boot_cr3 = PADDR(pgdir);
160
161	//////////////////////////////////////////////////////////////////////
162	// Recursively insert PD in itself as a page table, to form
163	// a virtual page table at virtual address VPT.
164	// (For now, you don't have understand the greater purpose of the
165	// following two lines.)
166
167	// Permissions: kernel RW, user NONE
168	pgdir[PDX(VPT)] = PADDR(pgdir)|PTE_W|PTE_P;
169
170	// same for UVPT
171	// Permissions: kernel R, user R 
172	pgdir[PDX(UVPT)] = PADDR(pgdir)|PTE_U|PTE_P;
173
174	//////////////////////////////////////////////////////////////////////
175	// Allocate an array of npage 'struct Page's and store it in 'pages'.
176	// The kernel uses this array to keep track of physical pages: for
177	// each physical page, there is a corresponding struct Page in this
178	// array.  'npage' is the number of physical pages in memory.
179	// User-level programs will get read-only access to the array as well.
180	// Your code goes here:
181	pages = boot_alloc(npage*sizeof(struct Page),PGSIZE);
182	//////////////////////////////////////////////////////////////////////
183	// Now that we've allocated the initial kernel data structures, we set
184	// up the list of free physical pages. Once we've done so, all further
185	// memory management will go through the page_* functions. In
186	// particular, we can now map memory using boot_map_segment or page_insert
187	page_init();
188
189	check_page_alloc();
190
191	page_check();
192
193	//////////////////////////////////////////////////////////////////////
194	// Now we set up virtual memory 
195
196	//////////////////////////////////////////////////////////////////////
197	// Map 'pages' read-only by the user at linear address UPAGES
198	// Permissions:
199	//    - the new image at UPAGES -- kernel R, user R
200	//      (ie. perm = PTE_U | PTE_P)
201	//    - pages itself -- kernel RW, user NONE
202	// Your code goes here:
203
204	//////////////////////////////////////////////////////////////////////
205	// Use the physical memory that 'bootstack' refers to as the kernel
206	// stack.  The kernel stack grows down from virtual address KSTACKTOP.
207	// We consider the entire range from [KSTACKTOP-PTSIZE, KSTACKTOP) 
208	// to be the kernel stack, but break this into two pieces:
209	//     * [KSTACKTOP-KSTKSIZE, KSTACKTOP) -- backed by physical memory
210	//     * [KSTACKTOP-PTSIZE, KSTACKTOP-KSTKSIZE) -- not backed; so if
211	//       the kernel overflows its stack, it will fault rather than
212	//       overwrite memory.  Known as a "guard page".
213	//     Permissions: kernel RW, user NONE
214	// Your code goes here:
215
216	//////////////////////////////////////////////////////////////////////
217	// Map all of physical memory at KERNBASE. 
218	// Ie.  the VA range [KERNBASE, 2^32) should map to
219	//      the PA range [0, 2^32 - KERNBASE)
220	// We might not have 2^32 - KERNBASE bytes of physical memory, but
221	// we just set up the mapping anyway.
222	// Permissions: kernel RW, user NONE
223	// Your code goes here: 
224
225	// Check that the initial page directory has been set up correctly.
226	check_boot_pgdir();
227
228	//////////////////////////////////////////////////////////////////////
229	// On x86, segmentation maps a VA to a LA (linear addr) and
230	// paging maps the LA to a PA.  I.e. VA => LA => PA.  If paging is
231	// turned off the LA is used as the PA.  Note: there is no way to
232	// turn off segmentation.  The closest thing is to set the base
233	// address to 0, so the VA => LA mapping is the identity.
234
235	// Current mapping: VA KERNBASE+x => PA x.
236	//     (segmentation base=-KERNBASE and paging is off)
237
238	// From here on down we must maintain this VA KERNBASE + x => PA x
239	// mapping, even though we are turning on paging and reconfiguring
240	// segmentation.
241
242	// Map VA 0:4MB same as VA KERNBASE, i.e. to PA 0:4MB.
243	// (Limits our kernel to <4MB)
244	pgdir[0] = pgdir[PDX(KERNBASE)];
245
246	// Install page table.
247	lcr3(boot_cr3);
248
249	// Turn on paging.
250	cr0 = rcr0();
251	cr0 |= CR0_PE|CR0_PG|CR0_AM|CR0_WP|CR0_NE|CR0_TS|CR0_EM|CR0_MP;
252	cr0 &= ~(CR0_TS|CR0_EM);
253	lcr0(cr0);
254
255	// Current mapping: KERNBASE+x => x => x.
256	// (x < 4MB so uses paging pgdir[0])
257
258	// Reload all segment registers.
259	asm volatile("lgdt gdt_pd");
260	asm volatile("movw %%ax,%%gs" :: "a" (GD_UD|3));
261	asm volatile("movw %%ax,%%fs" :: "a" (GD_UD|3));
262	asm volatile("movw %%ax,%%es" :: "a" (GD_KD));
263	asm volatile("movw %%ax,%%ds" :: "a" (GD_KD));
264	asm volatile("movw %%ax,%%ss" :: "a" (GD_KD));
265	asm volatile("ljmp %0,$1f\n 1:\n" :: "i" (GD_KT));  // reload cs
266	asm volatile("lldt %%ax" :: "a" (0));
267
268	// Final mapping: KERNBASE+x => KERNBASE+x => x.
269
270	// This mapping was only used after paging was turned on but
271	// before the segment registers were reloaded.
272	pgdir[0] = 0;
273
274	// Flush the TLB for good measure, to kill the pgdir[0] mapping.
275	lcr3(boot_cr3);
276}
277
278//
279// Check the physical page allocator (page_alloc(), page_free(),
280// and page_init()).
281//
282	static void
283check_page_alloc()
284{
285	struct Page *pp, *pp0, *pp1, *pp2;
286	struct Page_list fl;
287
288	// if there's a page that shouldn't be on
289	// the free list, try to make sure it
290	// eventually causes trouble.
291	LIST_FOREACH(pp0, &page_free_list, pp_link)
292		memset(page2kva(pp0), 0x97, 128);
293
294	LIST_FOREACH(pp0, &page_free_list, pp_link) {
295		// check that we didn't corrupt the free list itself
296		assert(pp0 >= pages);
297		assert(pp0 < pages + npage);
298
299		// check a few pages that shouldn't be on the free list
300		assert(page2pa(pp0) != 0);
301		assert(page2pa(pp0) != IOPHYSMEM);
302		assert(page2pa(pp0) != EXTPHYSMEM - PGSIZE);
303		assert(page2pa(pp0) != EXTPHYSMEM);
304		assert(page2kva(pp0) != ROUNDDOWN(boot_freemem - 1, PGSIZE));
305	}
306
307	// should be able to allocate three pages
308	pp0 = pp1 = pp2 = 0;
309	assert(page_alloc(&pp0) == 0);
310	assert(page_alloc(&pp1) == 0);
311	assert(page_alloc(&pp2) == 0);
312
313	assert(pp0);
314	assert(pp1 && pp1 != pp0);
315	assert(pp2 && pp2 != pp1 && pp2 != pp0);
316	assert(page2pa(pp0) < npage*PGSIZE);
317	assert(page2pa(pp1) < npage*PGSIZE);
318	assert(page2pa(pp2) < npage*PGSIZE);
319
320	// temporarily steal the rest of the free pages
321	fl = page_free_list;
322	LIST_INIT(&page_free_list);
323
324	// should be no free memory
325	assert(page_alloc(&pp) == -E_NO_MEM);
326
327	// free and re-allocate?
328	page_free(pp0);
329	page_free(pp1);
330	page_free(pp2);
331	pp0 = pp1 = pp2 = 0;
332	assert(page_alloc(&pp0) == 0);
333	assert(page_alloc(&pp1) == 0);
334	assert(page_alloc(&pp2) == 0);
335	assert(pp0);
336	assert(pp1 && pp1 != pp0);
337	assert(pp2 && pp2 != pp1 && pp2 != pp0);
338	assert(page_alloc(&pp) == -E_NO_MEM);
339
340	// give free list back
341	page_free_list = fl;
342
343	// free the pages we took
344	page_free(pp0);
345	page_free(pp1);
346	page_free(pp2);
347
348	cprintf("check_page_alloc() succeeded!\n");
349}
350
351//
352// Checks that the kernel part of virtual address space
353// has been setup roughly correctly(by i386_vm_init()).
354//
355// This function doesn't test every corner case,
356// in fact it doesn't test the permission bits at all,
357// but it is a pretty good sanity check. 
358//
359static physaddr_t check_va2pa(pde_t *pgdir, uintptr_t va);
360
361	static void
362check_boot_pgdir(void)
363{
364	uint32_t i, n;
365	pde_t *pgdir;
366
367	pgdir = boot_pgdir;
368
369	// check pages array
370	n = ROUNDUP(npage*sizeof(struct Page), PGSIZE);
371	for (i = 0; i < n; i += PGSIZE)
372		assert(check_va2pa(pgdir, UPAGES + i) == PADDR(pages) + i);
373
374
375	// check phys mem
376	for (i = 0; i < npage * PGSIZE; i += PGSIZE)
377		assert(check_va2pa(pgdir, KERNBASE + i) == i);
378
379	// check kernel stack
380	for (i = 0; i < KSTKSIZE; i += PGSIZE)
381		assert(check_va2pa(pgdir, KSTACKTOP - KSTKSIZE + i) == PADDR(bootstack) + i);
382	assert(check_va2pa(pgdir, KSTACKTOP - PTSIZE) == ~0);
383
384	// check for zero/non-zero in PDEs
385	for (i = 0; i < NPDENTRIES; i++) {
386		switch (i) {
387			case PDX(VPT):
388			case PDX(UVPT):
389			case PDX(KSTACKTOP-1):
390			case PDX(UPAGES):
391				assert(pgdir[i]);
392				break;
393			default:
394				if (i >= PDX(KERNBASE))
395					assert(pgdir[i]);
396				else
397					assert(pgdir[i] == 0);
398				break;
399		}
400	}
401	cprintf("check_boot_pgdir() succeeded!\n");
402}
403
404// This function returns the physical address of the page containing 'va',
405// defined by the page directory 'pgdir'.  The hardware normally performs
406// this functionality for us!  We define our own version to help check
407// the check_boot_pgdir() function; it shouldn't be used elsewhere.
408
409	static physaddr_t
410check_va2pa(pde_t *pgdir, uintptr_t va)
411{
412	pte_t *p;
413
414	pgdir = &pgdir[PDX(va)];
415	if (!(*pgdir & PTE_P))
416		return ~0;
417	p = (pte_t*) KADDR(PTE_ADDR(*pgdir));
418	if (!(p[PTX(va)] & PTE_P))
419		return ~0;
420	return PTE_ADDR(p[PTX(va)]);
421}
422
423// --------------------------------------------------------------
424// Tracking of physical pages.
425// The 'pages' array has one 'struct Page' entry per physical page.
426// Pages are reference counted, and free pages are kept on a linked list.
427// --------------------------------------------------------------
428
429//
430// Initialize page structure and memory free list.
431// After this is done, NEVER use boot_alloc again.  ONLY use the page
432// allocator functions below to allocate and deallocate physical
433// memory via the page_free_list.
434//
435	void
436page_init(void)
437{
438	// The example code here marks all physical pages as free.
439	// However this is not truly the case.  What memory is free?
440	//  1) Mark physical page 0 as in use.
441	//     This way we preserve the real-mode IDT and BIOS structures
442	//     in case we ever need them.  (Currently we don't, but...)
443	//  2) The rest of base memory, [PGSIZE, basemem) is free.
444	//  3) Then comes the IO hole [IOPHYSMEM, EXTPHYSMEM).
445	//     Mark it as in use so that it can never be allocated.
446	//  4) Then extended memory [EXTPHYSMEM, ...).
447	//     Some of it is in use, some is free. Where is the kernel
448	//     in physical memory?  Which pages are already in use for
449	//     page tables and other data structures?
450	//
451	// Change the code to reflect this.
452	int i;
453	LIST_INIT(&page_free_list);
454	int io_hole_beginning = IOPHYSMEM >> PGSHIFT;
455	int io_hole_end = EXTPHYSMEM >> PGSHIFT;
456	for (i = 1; i < npage; i++) {
457		pages[i].pp_ref = 0;
458		LIST_INSERT_HEAD(&page_free_list, &pages[i], pp_link);
459		if ( i == io_hole_beginning - 1)
460			i = i + io_hole_end + 32;
461	}
462}
463
464//
465// Initialize a Page structure.
466// The result has null links and 0 refcount.
467// Note that the corresponding physical page is NOT initialized!
468//
469	static void
470page_initpp(struct Page *pp)
471{
472	memset(pp, 0, sizeof(*pp));
473}
474
475//
476// Allocates a physical page.
477// Does NOT set the contents of the physical page to zero, NOR does it
478// increment the reference count of the page - the caller must do
479// these if necessary. 
480//
481// *pp_store -- is set to point to the Page struct of the newly allocated
482// page
483//
484// RETURNS 
485//   0 -- on success
486//   -E_NO_MEM -- otherwise 
487//
488// Hint: use LIST_FIRST, LIST_REMOVE, and page_initpp
489	int
490page_alloc(struct Page **pp_store)
491{
492	// Fill this function in
493	if(!LIST_EMPTY(&page_free_list))
494	{
495		//                page_initpp(*pp_store);
496
497		*pp_store = LIST_FIRST(&page_free_list);
498
499		LIST_REMOVE(*pp_store,pp_link);
500
501		page_initpp(*pp_store);                
502
503		return 0;
504	}
505	else
506		return -E_NO_MEM;
507}
508
509//
510// Return a page to the free list.
511// (This function should only be called when pp->pp_ref reaches 0.)
512//
513	void
514page_free(struct Page *pp)
515{
516	// Fill this function in
517	if(pp->pp_ref == 0)
518		LIST_INSERT_HEAD(&page_free_list, pp, pp_link);
519}
520
521//
522// Decrement the reference count on a page,
523// freeing it if there are no more refs.
524//
525	void
526page_decref(struct Page* pp)
527{
528	if (--pp->pp_ref == 0)
529		page_free(pp);
530}
531
532// Given 'pgdir', a pointer to a page directory, pgdir_walk returns
533// a pointer to the page table entry (PTE) for linear address 'va'.
534// This requires walking the two-level page table structure.
535//
536// If the relevant page table doesn't exist in the page directory, then:
537//    - If create == 0, pgdir_walk returns NULL.
538//    - Otherwise, pgdir_walk tries to allocate a new page table
539//	with page_alloc.  If this fails, pgdir_walk returns NULL.
540//    - pgdir_walk sets pp_ref to 1 for the new page table.
541//    - pgdir_walk clears the new page table.
542//    - Finally, pgdir_walk returns a pointer into the new page table.
543//
544// Hint: you can turn a Page * into the physical address of the
545// page it refers to with page2pa() from kern/pmap.h.
546//
547// Hint 2: the x86 MMU checks permission bits in both the page directory
548// and the page table, so it's safe to leave permissions in the page
549// more permissive than strictly necessary.
550	pte_t *
551pgdir_walk(pde_t *pgdir, const void *va, int create)
552{
553	// Fill this function in
554	pde_t *pg_dir_entry;
555	pte_t *pg_table_entry;
556	struct Page *pg;
557	pg_dir_entry = &pgdir[PDX(va)];
558	if( *pg_dir_entry & PTE_P )
559		pg_table_entry = (pte_t*)KADDR( PTE_ADDR(*pg_dir_entry));
560	else
561	{
562		if (create == 0)
563			return NULL;
564		if(page_alloc(&pg) == -E_NO_MEM)
565			return NULL;
566		pg->pp_ref=1;
567		pg_table_entry = (pte_t*)page2kva(pg);
568		*pg_dir_entry = page2pa(pg)|PTE_P|PTE_W|PTE_U;
569	}
570	return &pg_table_entry[PTX(va)];
571}
572
573//
574// Map the physical page 'pp' at virtual address 'va'.
575// The permissions (the low 12 bits) of the page table
576//  entry should be set to 'perm|PTE_P'.
577//
578// Requirements
579//   - If there is already a page mapped at 'va', it should be page_remove()d.
580//   - If necessary, on demand, a page table should be allocated and inserted
581//     into 'pgdir'.
582//   - pp->pp_ref should be incremented if the insertion succeeds.
583//   - The TLB must be invalidated if a page was formerly present at 'va'.
584//
585// Corner-case hint: Make sure to consider what happens when the same 
586// pp is re-inserted at the same virtual address in the same pgdir.
587//
588// RETURNS: 
589//   0 on success
590//   -E_NO_MEM, if page table couldn't be allocated
591//
592// Hint: The TA solution is implemented using pgdir_walk, page_remove,
593// and page2pa.
594//
595	int
596page_insert(pde_t *pgdir, struct Page *pp, void *va, int perm) 
597{
598	// Fill this function in
599	pte_t *pte;
600	pte = pgdir_walk(pgdir,va,1);
601	if(pte == NULL)
602		return -E_NO_MEM;
603	if(check_va2pa(pgdir, (uintptr_t)va) == page2pa(pp)){
604
605		*pte = page2pa(pp)|perm|PTE_P;
606		return 0;
607	}
608	if(*pte & PTE_P)
609		page_remove(pgdir,va);
610	pp->pp_ref++;
611	*pte = page2pa(pp)|perm|PTE_P;
612	return 0;
613}
614
615//
616// Map [la, la+size) of linear address space to physical [pa, pa+size)
617// in the page table rooted at pgdir.  Size is a multiple of PGSIZE.
618// Use permission bits perm|PTE_P for the entries.
619//
620// This function is only intended to set up the ``static'' mappings
621// above UTOP. As such, it should *not* change the pp_ref field on the
622// mapped pages.
623//
624// Hint: the TA solution uses pgdir_walk
625	static void
626boot_map_segment(pde_t *pgdir, uintptr_t la, size_t size, physaddr_t pa, int perm)
627{
628	// Fill this function in
629	uintptr_t i;
630	for (i=0; i<size; i+=PGSIZE)
631	{
632		*pgdir_walk(pgdir,(uintptr_t *)(la+i), 1) = (pa+i)|perm|PTE_P;
633	}
634}
635
636//
637// Return the page mapped at virtual address 'va'.
638// If pte_store is not zero, then we store in it the address
639// of the pte for this page.  This is used by page_remove and
640// can be used to verify page permissions for syscall arguments,
641// but should not be used by most callers.
642//
643// Return NULL if there is no page mapped at va.
644//
645// Hint: the TA solution uses pgdir_walk and pa2page.
646//
647	struct Page *
648page_lookup(pde_t *pgdir, void *va, pte_t **pte_store)
649{
650	// Fill this function in
651	pte_t *pte;
652	pte = pgdir_walk(pgdir, va, 0);
653	if(pte == NULL)
654		return NULL;
655	if(**pte_store != 0)
656	{
657		*pte_store = pte;
658		return pa2page(**pte_store);
659	}
660	else
661		return NULL;
662}
663
664//
665// Unmaps the physical page at virtual address 'va'.
666// If there is no physical page at that address, silently does nothing.
667//
668// Details:
669//   - The ref count on the physical page should decrement.
670//   - The physical page should be freed if the refcount reaches 0.
671//   - The pg table entry corresponding to 'va' should be set to 0.
672//     (if such a PTE exists)
673//   - The TLB must be invalidated if you remove an entry from
674//     the pg dir/pg table.
675//
676// Hint: The TA solution is implemented using page_lookup,
677// 	tlb_invalidate, and page_decref.
678//
679	void
680page_remove(pde_t *pgdir, void *va)
681{
682	// Fill this function in
683	struct Page *pg;
684	pte_t *pte;
685	pg = page_lookup(pgdir,va,&pte);
686	if( pg != NULL)
687	{
688		page_decref(pg);
689		if(*pte & PTE_P)
690			*pte = 0;
691		tlb_invalidate(pgdir,va);
692	}
693}
694
695//
696// Invalidate a TLB entry, but only if the page tables being
697// edited are the ones currently in use by the processor.
698//
699	void
700tlb_invalidate(pde_t *pgdir, void *va)
701{
702	// Flush the entry only if we're modifying the current address space.
703	// For now, there is only one address space, so always invalidate.
704	invlpg(va);
705}
706
707// check page_insert, page_remove, &c
708	static void
709page_check(void)
710{
711	struct Page *pp, *pp0, *pp1, *pp2;
712	struct Page_list fl;
713	pte_t *ptep, *ptep1;
714	void *va;
715	int i;
716
717	// should be able to allocate three pages
718	pp0 = pp1 = pp2 = 0;
719	assert(page_alloc(&pp0) == 0);
720	assert(page_alloc(&pp1) == 0);
721	assert(page_alloc(&pp2) == 0);
722
723	assert(pp0);
724	assert(pp1 && pp1 != pp0);
725	assert(pp2 && pp2 != pp1 && pp2 != pp0);
726
727	// temporarily steal the rest of the free pages
728	fl = page_free_list;
729	LIST_INIT(&page_free_list);
730
731	// should be no free memory
732	assert(page_alloc(&pp) == -E_NO_MEM);
733
734	// there is no page allocated at address 0
735	assert(page_lookup(boot_pgdir, (void *) 0x0, &ptep) == NULL);
736
737	// there is no free memory, so we can't allocate a page table 
738	assert(page_insert(boot_pgdir, pp1, 0x0, 0) < 0);
739
740	// free pp0 and try again: pp0 should be used for page table
741	page_free(pp0);
742	assert(page_insert(boot_pgdir, pp1, 0x0, 0) == 0);
743	assert(PTE_ADDR(boot_pgdir[0]) == page2pa(pp0));
744	assert(check_va2pa(boot_pgdir, 0x0) == page2pa(pp1));
745	assert(pp1->pp_ref == 1);
746	assert(pp0->pp_ref == 1);
747
748	// should be able to map pp2 at PGSIZE because pp0 is already allocated for page table
749
750	assert(page_insert(boot_pgdir, pp2, (void*) PGSIZE, 0) == 0);
751	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp2));
752	assert(pp2->pp_ref == 1);
753
754	// should be no free memory
755	assert(page_alloc(&pp) == -E_NO_MEM);
756
757	// should be able to map pp2 at PGSIZE because it's already there
758	assert(page_insert(boot_pgdir, pp2, (void*) PGSIZE, 0) == 0);
759	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp2));
760	assert(pp2->pp_ref == 1);
761
762	// pp2 should NOT be on the free list
763	// could happen in ref counts are handled sloppily in page_insert
764	assert(page_alloc(&pp) == -E_NO_MEM);
765
766	// check that pgdir_walk returns a pointer to the pte
767	ptep = KADDR(PTE_ADDR(boot_pgdir[PDX(PGSIZE)]));
768	assert(pgdir_walk(boot_pgdir, (void*)PGSIZE, 0) == ptep+PTX(PGSIZE));
769
770	// should be able to change permissions too.
771	assert(page_insert(boot_pgdir, pp2, (void*) PGSIZE, PTE_U) == 0);
772	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp2));
773	assert(pp2->pp_ref == 1);
774	assert(*pgdir_walk(boot_pgdir, (void*) PGSIZE, 0) & PTE_U);
775	assert(boot_pgdir[0] & PTE_U);
776
777	// should not be able to map at PTSIZE because need free page for page table
778	assert(page_insert(boot_pgdir, pp0, (void*) PTSIZE, 0) < 0);
779
780	// insert pp1 at PGSIZE (replacing pp2)
781	assert(page_insert(boot_pgdir, pp1, (void*) PGSIZE, 0) == 0);
782	assert(!(*pgdir_walk(boot_pgdir, (void*) PGSIZE, 0) & PTE_U));
783
784	// should have pp1 at both 0 and PGSIZE, pp2 nowhere, ...
785	assert(check_va2pa(boot_pgdir, 0) == page2pa(pp1));
786	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp1));
787	// ... and ref counts should reflect this
788	assert(pp1->pp_ref == 2);
789	assert(pp2->pp_ref == 0);
790
791	// pp2 should be returned by page_alloc
792	assert(page_alloc(&pp) == 0 && pp == pp2);
793
794	// unmapping pp1 at 0 should keep pp1 at PGSIZE
795	page_remove(boot_pgdir, 0x0);
796	assert(check_va2pa(boot_pgdir, 0x0) == ~0);
797	assert(check_va2pa(boot_pgdir, PGSIZE) == page2pa(pp1));
798	assert(pp1->pp_ref == 1);
799	assert(pp2->pp_ref == 0);
800
801	// unmapping pp1 at PGSIZE should free it
802	page_remove(boot_pgdir, (void*) PGSIZE);
803	assert(check_va2pa(boot_pgdir, 0x0) == ~0);
804	assert(check_va2pa(boot_pgdir, PGSIZE) == ~0);
805	assert(pp1->pp_ref == 0);
806	assert(pp2->pp_ref == 0);
807
808	// so it should be returned by page_alloc
809	assert(page_alloc(&pp) == 0 && pp == pp1);
810
811	// should be no free memory
812	assert(page_alloc(&pp) == -E_NO_MEM);
813
814#if 0
815	// should be able to page_insert to change a page
816	// and see the new data immediately.
817	memset(page2kva(pp1), 1, PGSIZE);
818	memset(page2kva(pp2), 2, PGSIZE);
819	page_insert(boot_pgdir, pp1, 0x0, 0);
820	assert(pp1->pp_ref == 1);
821	assert(*(int*)0 == 0x01010101);
822	page_insert(boot_pgdir, pp2, 0x0, 0);
823	assert(*(int*)0 == 0x02020202);
824	assert(pp2->pp_ref == 1);
825	assert(pp1->pp_ref == 0);
826	page_remove(boot_pgdir, 0x0);
827	assert(pp2->pp_ref == 0);
828#endif
829
830	// forcibly take pp0 back
831	assert(PTE_ADDR(boot_pgdir[0]) == page2pa(pp0));
832	boot_pgdir[0] = 0;
833	assert(pp0->pp_ref == 1);
834	pp0->pp_ref = 0;
835
836	// check pointer arithmetic in pgdir_walk
837	page_free(pp0);
838	va = (void*)(PGSIZE * NPDENTRIES + PGSIZE);
839	ptep = pgdir_walk(boot_pgdir, va, 1);
840	ptep1 = KADDR(PTE_ADDR(boot_pgdir[PDX(va)]));
841	assert(ptep == ptep1 + PTX(va));
842	boot_pgdir[PDX(va)] = 0;
843	pp0->pp_ref = 0;
844
845	// check that new page tables get cleared
846	memset(page2kva(pp0), 0xFF, PGSIZE);
847	page_free(pp0);
848	pgdir_walk(boot_pgdir, 0x0, 1);
849	ptep = page2kva(pp0);
850	for(i=0; i<NPTENTRIES; i++)
851		assert((ptep[i] & PTE_P) == 0);
852	boot_pgdir[0] = 0;
853	pp0->pp_ref = 0;
854
855	// give free list back
856	page_free_list = fl;
857
858	// free the pages we took
859	page_free(pp0);
860	page_free(pp1);
861	page_free(pp2);
862
863	cprintf("page_check() succeeded!\n");
864}