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/*

 * Memory mappings.  Life was easier when 2G of memory was enough.

 *

 * The kernel memory starts at KZERO, with the text loaded at KZERO+1M

 * (9load sits under 1M during the load).  The memory from KZERO to the

 * top of memory is mapped 1-1 with physical memory, starting at physical

 * address 0.  All kernel memory and data structures (i.e., the entries stored

 * into conf.mem) must sit in this physical range: if KZERO is at 0xF0000000,

 * then the kernel can only have 256MB of memory for itself.

 * 

 * The 256M below KZERO comprises three parts.  The lowest 4M is the

 * virtual page table, a virtual address representation of the current 

 * page table tree.  The second 4M is used for temporary per-process

 * mappings managed by kmap and kunmap.  The remaining 248M is used

 * for global (shared by all procs and all processors) device memory

 * mappings and managed by vmap and vunmap.  The total amount (256M)

 * could probably be reduced somewhat if desired.  The largest device

 * mapping is that of the video card, and even though modern video cards

 * have embarrassing amounts of memory, the video drivers only use one

 * frame buffer worth (at most 16M).  Each is described in more detail below.

 *

 * The VPT is a 4M frame constructed by inserting the pdb into itself.

 * This short-circuits one level of the page tables, with the result that 

 * the contents of second-level page tables can be accessed at VPT.  

 * We use the VPT to edit the page tables (see mmu) after inserting them

 * into the page directory.  It is a convenient mechanism for mapping what

 * might be otherwise-inaccessible pages.  The idea was borrowed from

 * the Exokernel.

 *

 * The VPT doesn't solve all our problems, because we still need to 

 * prepare page directories before we can install them.  For that, we

 * use tmpmap/tmpunmap, which map a single page at TMPADDR.

 */

#include	"u.h"

#include	"../port/lib.h"

#include	"mem.h"

#include	"dat.h"

#include	"fns.h"

#include	"io.h"

/*

 * Simple segment descriptors with no translation.

 */

#define	DATASEGM(p) 	{ 0xFFFF, SEGG|SEGB|(0xF<<16)|SEGP|SEGPL(p)|SEGDATA|SEGW }

#define	EXECSEGM(p) 	{ 0xFFFF, SEGG|SEGD|(0xF<<16)|SEGP|SEGPL(p)|SEGEXEC|SEGR }

#define	EXEC16SEGM(p) 	{ 0xFFFF, SEGG|(0xF<<16)|SEGP|SEGPL(p)|SEGEXEC|SEGR }

#define	TSSSEGM(b,p)	{ ((b)<<16)|sizeof(Tss),\

			  ((b)&0xFF000000)|(((b)>>16)&0xFF)|SEGTSS|SEGPL(p)|SEGP }

void realmodeintrinst(void);

void _stop32pg(void);

Segdesc gdt[NGDT] =

{

[NULLSEG]	{ 0, 0},		/* null descriptor */

[KDSEG]		DATASEGM(0),		/* kernel data/stack */

[KESEG]		EXECSEGM(0),		/* kernel code */

[UDSEG]		DATASEGM(3),		/* user data/stack */

[UESEG]		EXECSEGM(3),		/* user code */

[TSSSEG]	TSSSEGM(0,0),		/* tss segment */

[KESEG16]		EXEC16SEGM(0),	/* kernel code 16-bit */

};

static int didmmuinit;

static void taskswitch(ulong, ulong);

static void memglobal(void);

#define	vpt ((ulong*)VPT)

#define	VPTX(va)		(((ulong)(va))>>12)

#define	vpd (vpt+VPTX(VPT))

void

mmuinit0(void)

{

	memmove(m->gdt, gdt, sizeof gdt);

}

void

mmuinit(void)

{

	ulong x, *p;

	ushort ptr[3];

	didmmuinit = 1;

	if(0) print("vpt=%#.8ux vpd=%#p kmap=%#.8ux\n",

		VPT, vpd, KMAP);

	memglobal();

	m->pdb[PDX(VPT)] = PADDR(m->pdb)|PTEWRITE|PTEVALID;

	m->tss = malloc(sizeof(Tss));

	if(m->tss == nil)

		panic("mmuinit: no memory");

	memset(m->tss, 0, sizeof(Tss));

	m->tss->iomap = 0xDFFF<<16;

	/*

	 * We used to keep the GDT in the Mach structure, but it

	 * turns out that that slows down access to the rest of the

	 * page.  Since the Mach structure is accessed quite often,

	 * it pays off anywhere from a factor of 1.25 to 2 on real

	 * hardware to separate them (the AMDs are more sensitive

	 * than Intels in this regard).  Under VMware it pays off

	 * a factor of about 10 to 100.

	 */

	memmove(m->gdt, gdt, sizeof gdt);

	x = (ulong)m->tss;

	m->gdt[TSSSEG].d0 = (x<<16)|sizeof(Tss);

	m->gdt[TSSSEG].d1 = (x&0xFF000000)|((x>>16)&0xFF)|SEGTSS|SEGPL(0)|SEGP;

	ptr[0] = sizeof(gdt)-1;

	x = (ulong)m->gdt;

	ptr[1] = x & 0xFFFF;

	ptr[2] = (x>>16) & 0xFFFF;

	lgdt(ptr);

	ptr[0] = sizeof(Segdesc)*256-1;

	x = IDTADDR;

	ptr[1] = x & 0xFFFF;

	ptr[2] = (x>>16) & 0xFFFF;

	lidt(ptr);

	/*

	 * this kills 9load but not 9boot.  9load dies at the taskswitch.

	 * should track down exactly why some day.

	 */

	/* make most kernel text unwritable */

if(0)	for(x = PGROUND((ulong)_stop32pg); x < (ulong)etext; x += BY2PG){

		if (x == (ulong)realmodeintrinst & ~(BY2PG-1))

			continue;

		p = mmuwalk(m->pdb, x, 2, 0);

		if(p == nil)

			panic("mmuinit");

		*p &= ~PTEWRITE;

	}

	taskswitch(PADDR(m->pdb), (ulong)m + MACHSIZE);

	ltr(TSSSEL);

}

/* 

 * On processors that support it, we set the PTEGLOBAL bit in

 * page table and page directory entries that map kernel memory.

 * Doing this tells the processor not to bother flushing them

 * from the TLB when doing the TLB flush associated with a 

 * context switch (write to CR3).  Since kernel memory mappings

 * are never removed, this is safe.  (If we ever remove kernel memory

 * mappings, we can do a full flush by turning off the PGE bit in CR4,

 * writing to CR3, and then turning the PGE bit back on.) 

 *

 * See also mmukmap below.

 * 

 * Processor support for the PTEGLOBAL bit is enabled in devarch.c.

 */

static void

memglobal(void)

{

	int i, j;

	ulong *pde, *pte;

	/* only need to do this once, on bootstrap processor */

	if(m->machno != 0)

		return;

	if(!m->havepge)

		return;

	pde = m->pdb;

	for(i=PDX(KZERO); i<1024; i++){

		if(pde[i] & PTEVALID){

			pde[i] |= PTEGLOBAL;

			if(!(pde[i] & PTESIZE)){

				pte = KADDR(pde[i]&~(BY2PG-1));

				for(j=0; j<1024; j++)

					if(pte[j] & PTEVALID)

						pte[j] |= PTEGLOBAL;

			}

		}

	}			

}

/*

 * Flush all the user-space and device-mapping mmu info

 * for this process, because something has been deleted.

 * It will be paged back in on demand.

 */

void

flushmmu(void)

{

	int s;

	s = splhi();

	up->newtlb = 1;

	mmuswitch(up);

	splx(s);

}

/*

 * Flush a single page mapping from the tlb.

 */

void

flushpg(ulong va)

{

	if(X86FAMILY(m->cpuidax) >= 4)

		invlpg(va);

	else

		putcr3(getcr3());

}

/*

 * Allocate a new page for a page directory. 

 * We keep a small cache of pre-initialized

 * page directories in each mach.

 */

static Page*

mmupdballoc(void)

{

	int s;

	Page *page;

	ulong *pdb;

	s = splhi();

	m->pdballoc++;

	if(m->pdbpool == 0){

		spllo();

		page = newpage(0, 0, 0);

		page->va = (ulong)vpd;

		splhi();

		pdb = tmpmap(page);

		memmove(pdb, m->pdb, BY2PG);

		pdb[PDX(VPT)] = page->pa|PTEWRITE|PTEVALID;	/* set up VPT */

		tmpunmap(pdb);

	}else{

		page = m->pdbpool;

		m->pdbpool = page->next;

		m->pdbcnt--;

	}

	splx(s);

	return page;

}

static void

mmupdbfree(Proc *proc, Page *p)

{

	if(islo())

		panic("mmupdbfree: islo");

	m->pdbfree++;

	if(m->pdbcnt >= 10){

		p->next = proc->mmufree;

		proc->mmufree = p;

	}else{

		p->next = m->pdbpool;

		m->pdbpool = p;

		m->pdbcnt++;

	}

}

/*

 * A user-space memory segment has been deleted, or the

 * process is exiting.  Clear all the pde entries for user-space

 * memory mappings and device mappings.  Any entries that

 * are needed will be paged back in as necessary.

 */

static void

mmuptefree(Proc* proc)

{

	int s;

	ulong *pdb;

	Page **last, *page;

	if(proc->mmupdb == nil || proc->mmuused == nil)

		return;

	s = splhi();

	pdb = tmpmap(proc->mmupdb);

	last = &proc->mmuused;

	for(page = *last; page; page = page->next){

		pdb[page->daddr] = 0;

		last = &page->next;

	}

	tmpunmap(pdb);

	splx(s);

	*last = proc->mmufree;

	proc->mmufree = proc->mmuused;

	proc->mmuused = 0;

}

static void

taskswitch(ulong pdb, ulong stack)

{

	Tss *tss;

	tss = m->tss;

	tss->ss0 = KDSEL;

	tss->esp0 = stack;

	tss->ss1 = KDSEL;

	tss->esp1 = stack;

	tss->ss2 = KDSEL;

	tss->esp2 = stack;

	putcr3(pdb);

}

void

mmuswitch(Proc* proc)

{

	ulong *pdb;

	if(proc->newtlb){

		mmuptefree(proc);

		proc->newtlb = 0;

	}

	if(proc->mmupdb){

		pdb = tmpmap(proc->mmupdb);

		pdb[PDX(MACHADDR)] = m->pdb[PDX(MACHADDR)];

		tmpunmap(pdb);

		taskswitch(proc->mmupdb->pa, (ulong)(proc->kstack+KSTACK));

	}else

		taskswitch(PADDR(m->pdb), (ulong)(proc->kstack+KSTACK));

}

/*

 * Release any pages allocated for a page directory base or page-tables

 * for this process:

 *   switch to the prototype pdb for this processor (m->pdb);

 *   call mmuptefree() to place all pages used for page-tables (proc->mmuused)

 *   onto the process' free list (proc->mmufree). This has the side-effect of

 *   cleaning any user entries in the pdb (proc->mmupdb);

 *   if there's a pdb put it in the cache of pre-initialised pdb's

 *   for this processor (m->pdbpool) or on the process' free list;

 *   finally, place any pages freed back into the free pool (palloc).

 * This routine is only called from schedinit() with palloc locked.

 */

void

mmurelease(Proc* proc)

{

	Page *page, *next;

	ulong *pdb;

	if(islo())

		panic("mmurelease: islo");

	taskswitch(PADDR(m->pdb), (ulong)m + BY2PG);

	if(proc->kmaptable){

		if(proc->mmupdb == nil)

			panic("mmurelease: no mmupdb");

		if(--proc->kmaptable->ref)

			panic("mmurelease: kmap ref %d", proc->kmaptable->ref);

		if(proc->nkmap)

			panic("mmurelease: nkmap %d", proc->nkmap);

		/*

		 * remove kmaptable from pdb before putting pdb up for reuse.

		 */

		pdb = tmpmap(proc->mmupdb);

		if(PPN(pdb[PDX(KMAP)]) != proc->kmaptable->pa)

			panic("mmurelease: bad kmap pde %#.8lux kmap %#.8lux",

				pdb[PDX(KMAP)], proc->kmaptable->pa);

		pdb[PDX(KMAP)] = 0;

		tmpunmap(pdb);

		/*

		 * move kmaptable to free list.

		 */

		pagechainhead(proc->kmaptable);

		proc->kmaptable = 0;

	}

	if(proc->mmupdb){

		mmuptefree(proc);

		mmupdbfree(proc, proc->mmupdb);

		proc->mmupdb = 0;

	}

	for(page = proc->mmufree; page; page = next){

		next = page->next;

		if(--page->ref)

			panic("mmurelease: page->ref %d", page->ref);

		pagechainhead(page);

	}

	if(proc->mmufree && palloc.r.p)

		wakeup(&palloc.r);

	proc->mmufree = 0;

}

/*

 * Allocate and install pdb for the current process.

 */

static void

upallocpdb(void)

{

	int s;

	ulong *pdb;

	Page *page;

	if(up->mmupdb != nil)

		return;

	page = mmupdballoc();

	s = splhi();

	if(up->mmupdb != nil){

		/*

		 * Perhaps we got an interrupt while

		 * mmupdballoc was sleeping and that

		 * interrupt allocated an mmupdb?

		 * Seems unlikely.

		 */

		mmupdbfree(up, page);

		splx(s);

		return;

	}

	pdb = tmpmap(page);

	pdb[PDX(MACHADDR)] = m->pdb[PDX(MACHADDR)];

	tmpunmap(pdb);

	up->mmupdb = page;

	putcr3(up->mmupdb->pa);

	splx(s);

}

/*

 * Update the mmu in response to a user fault.  pa may have PTEWRITE set.

 */

void

putmmu(ulong va, ulong pa, Page*)

{

	int old, s;

	Page *page;

	if(up->mmupdb == nil)

		upallocpdb();

	/*

	 * We should be able to get through this with interrupts

	 * turned on (if we get interrupted we'll just pick up 

	 * where we left off) but we get many faults accessing

	 * vpt[] near the end of this function, and they always happen

	 * after the process has been switched out and then 

	 * switched back, usually many times in a row (perhaps

	 * it cannot switch back successfully for some reason).

	 * 

	 * In any event, I'm tired of searching for this bug.  

	 * Turn off interrupts during putmmu even though

	 * we shouldn't need to.		- rsc

	 */

	s = splhi();

	if(!(vpd[PDX(va)]&PTEVALID)){

		if(up->mmufree == 0){

			spllo();

			page = newpage(0, 0, 0);

			splhi();

		}

		else{

			page = up->mmufree;

			up->mmufree = page->next;

		}

		vpd[PDX(va)] = PPN(page->pa)|PTEUSER|PTEWRITE|PTEVALID;

		/* page is now mapped into the VPT - clear it */

		memset((void*)(VPT+PDX(va)*BY2PG), 0, BY2PG);

		page->daddr = PDX(va);

		page->next = up->mmuused;

		up->mmuused = page;

	}

	old = vpt[VPTX(va)];

	vpt[VPTX(va)] = pa|PTEUSER|PTEVALID;

	if(old&PTEVALID)

		flushpg(va);

	if(getcr3() != up->mmupdb->pa)

		print("bad cr3 %#.8lux %#.8lux\n", getcr3(), up->mmupdb->pa);

	splx(s);

}

/*

 * Double-check the user MMU.

 * Error checking only.

 */

void

checkmmu(ulong va, ulong pa)

{

	if(up->mmupdb == 0)

		return;

	if(!(vpd[PDX(va)]&PTEVALID) || !(vpt[VPTX(va)]&PTEVALID))

		return;

	if(PPN(vpt[VPTX(va)]) != pa)

		print("%ld %s: va=%#08lux pa=%#08lux pte=%#08lux\n",

			up->pid, up->text,

			va, pa, vpt[VPTX(va)]);

}

/*

 * Walk the page-table pointed to by pdb and return a pointer

 * to the entry for virtual address va at the requested level.

 * If the entry is invalid and create isn't requested then bail

 * out early. Otherwise, for the 2nd level walk, allocate a new

 * page-table page and register it in the 1st level.  This is used

 * only to edit kernel mappings, which use pages from kernel memory,

 * so it's okay to use KADDR to look at the tables.

 */

ulong*

mmuwalk(ulong* pdb, ulong va, int level, int create)

{

	ulong *table;

	void *map;

	table = &pdb[PDX(va)];

	if(!(*table & PTEVALID) && create == 0)

		return 0;

	switch(level){

	default:

		return 0;

	case 1:

		return table;

	case 2:

		if(*table & PTESIZE)

			panic("mmuwalk2: va %luX entry %luX", va, *table);

		if(!(*table & PTEVALID)){

			/*

			 * Have to call low-level allocator from

			 * memory.c if we haven't set up the xalloc

			 * tables yet.

			 */

			if(didmmuinit)

				map = xspanalloc(BY2PG, BY2PG, 0);

			else

				map = rampage();

			if(map == nil)

				panic("mmuwalk xspanalloc failed");

			*table = PADDR(map)|PTEWRITE|PTEVALID;

		}

		table = KADDR(PPN(*table));

		return &table[PTX(va)];

	}

}

/*

 * Device mappings are shared by all procs and processors and

 * live in the virtual range VMAP to VMAP+VMAPSIZE.  The master

 * copy of the mappings is stored in mach0->pdb, and they are

 * paged in from there as necessary by vmapsync during faults.

 */

static Lock vmaplock;

static int findhole(ulong *a, int n, int count);

static ulong vmapalloc(ulong size);

static void pdbunmap(ulong*, ulong, int);

/*

 * Add a device mapping to the vmap range.

 */

void*

vmap(ulong pa, int size)

{

	int osize;

	ulong o, va;

	/*

	 * might be asking for less than a page.

	 */

	osize = size;

	o = pa & (BY2PG-1);

	pa -= o;

	size += o;

	size = ROUND(size, BY2PG);

	if(pa == 0){

		print("vmap pa=0 pc=%#p\n", getcallerpc(&pa));

		return nil;

	}

	ilock(&vmaplock);

	if((va = vmapalloc(size)) == 0 

	|| pdbmap(MACHP(0)->pdb, pa|PTEUNCACHED|PTEWRITE, va, size) < 0){

		iunlock(&vmaplock);

		return 0;

	}

	iunlock(&vmaplock);

	/* avoid trap on local processor

	for(i=0; i<size; i+=4*MB)

		vmapsync(va+i);

	*/

	USED(osize);

//	print("  vmap %#.8lux %d => %#.8lux\n", pa+o, osize, va+o);

	return (void*)(va + o);

}

static int

findhole(ulong *a, int n, int count)

{

	int have, i;

	have = 0;

	for(i=0; i<n; i++){

		if(a[i] == 0)

			have++;

		else

			have = 0;

		if(have >= count)

			return i+1 - have;

	}

	return -1;

}

/*

 * Look for free space in the vmap.

 */

static ulong

vmapalloc(ulong size)

{

	int i, n, o;

	ulong *vpdb;

	int vpdbsize;

	vpdb = &MACHP(0)->pdb[PDX(VMAP)];

	vpdbsize = VMAPSIZE/(4*MB);

	if(size >= 4*MB){

		n = (size+4*MB-1) / (4*MB);

		if((o = findhole(vpdb, vpdbsize, n)) != -1)

			return VMAP + o*4*MB;

		return 0;

	}

	n = (size+BY2PG-1) / BY2PG;

	for(i=0; i<vpdbsize; i++)

		if((vpdb[i]&PTEVALID) && !(vpdb[i]&PTESIZE))

			if((o = findhole(KADDR(PPN(vpdb[i])), WD2PG, n)) != -1)

				return VMAP + i*4*MB + o*BY2PG;

	if((o = findhole(vpdb, vpdbsize, 1)) != -1)

		return VMAP + o*4*MB;

	/*

	 * could span page directory entries, but not worth the trouble.

	 * not going to be very much contention.

	 */

	return 0;

}

/*

 * Remove a device mapping from the vmap range.

 * Since pdbunmap does not remove page tables, just entries,

 * the call need not be interlocked with vmap.

 */

void

vunmap(void *v, int size)

{

	int i;

	ulong va, o;

	Mach *nm;

	Proc *p;

	/*

	 * might not be aligned

	 */

	va = (ulong)v;

	o = va&(BY2PG-1);

	va -= o;

	size += o;

	size = ROUND(size, BY2PG);

	if(size < 0 || va < VMAP || va+size > VMAP+VMAPSIZE)

		panic("vunmap va=%#.8lux size=%#x pc=%#.8lux",

			va, size, getcallerpc(&v));

	pdbunmap(MACHP(0)->pdb, va, size);

	/*

	 * Flush mapping from all the tlbs and copied pdbs.

	 * This can be (and is) slow, since it is called only rarely.

	 * It is possible for vunmap to be called with up == nil,

	 * e.g. from the reset/init driver routines during system

	 * boot. In that case it suffices to flush the MACH(0) TLB

	 * and return.

	 */

	if(!active.thunderbirdsarego){

		if(MACHP(0)->pdb == 0)

			panic("vunmap: nil m->pdb pc=%#p", getcallerpc(&v));

		if(PADDR(MACHP(0)->pdb) == 0)

			panic("vunmap: nil PADDR(m->pdb)");

		putcr3(PADDR(MACHP(0)->pdb));

		return;

	}

	for(i=0; i<conf.nproc; i++){

		p = proctab(i);

		if(p->state == Dead)

			continue;

		if(p != up)

			p->newtlb = 1;

	}

	for(i=0; i<conf.nmach; i++){

		nm = MACHP(i);

		if(nm != m)

			nm->flushmmu = 1;

	}

	flushmmu();

	for(i=0; i<conf.nmach; i++){

		nm = MACHP(i);

		if(nm != m)

			while((active.machs&(1<<nm->machno)) && nm->flushmmu)

				;

	}

}

/*

 * Add kernel mappings for va -> pa for a section of size bytes.

 */

int

pdbmap(ulong *pdb, ulong pa, ulong va, int size)

{

	int pse;

	ulong pgsz, *pte, *table;

	ulong flag, off;

	flag = pa&0xFFF;

	pa &= ~0xFFF;

	if((MACHP(0)->cpuiddx & 0x08) && (getcr4() & 0x10))

		pse = 1;

	else

		pse = 0;

	for(off=0; off<size; off+=pgsz){

		table = &pdb[PDX(va+off)];

		if((*table&PTEVALID) && (*table&PTESIZE))

			panic("vmap: pdb pte valid and big page: "

				"va=%#.8lux pa=%#.8lux pde=%#.8lux",

				va+off, pa+off, *table);

		/*

		 * Check if it can be mapped using a 4MB page:

		 * va, pa aligned and size >= 4MB and processor can do it.

		 */

		if(pse && (pa+off)%(4*MB) == 0 && (va+off)%(4*MB) == 0 &&

		    (size-off) >= 4*MB){

			*table = (pa+off)|flag|PTESIZE|PTEVALID;

			pgsz = 4*MB;

		}else{

			pte = mmuwalk(pdb, va+off, 2, 1);

			if(*pte&PTEVALID)

				panic("vmap: va=%#.8lux pa=%#.8lux pte=%#.8lux",

					va+off, pa+off, *pte);

			*pte = (pa+off)|flag|PTEVALID;

			pgsz = BY2PG;

		}

	}

	return 0;

}

/*

 * Remove mappings.  Must already exist, for sanity.

 * Only used for kernel mappings, so okay to use KADDR.

 */

static void

pdbunmap(ulong *pdb, ulong va, int size)

{

	ulong vae;

	ulong *table;

	vae = va+size;

	while(va < vae){

		table = &pdb[PDX(va)];

		if(!(*table & PTEVALID)){

			panic("vunmap: not mapped");

			/* 

			va = (va+4*MB-1) & ~(4*MB-1);

			continue;

			*/

		}

		if(*table & PTESIZE){

			*table = 0;

			va = (va+4*MB-1) & ~(4*MB-1);

			continue;

		}

		table = KADDR(PPN(*table));

		if(!(table[PTX(va)] & PTEVALID))

			panic("vunmap: not mapped");

		table[PTX(va)] = 0;

		va += BY2PG;

	}

}

/*

 * Handle a fault by bringing vmap up to date.

 * Only copy pdb entries and they never go away,

 * so no locking needed.

 */

int

vmapsync(ulong va)

{

	ulong entry, *table;

	if(va < VMAP || va >= VMAP+VMAPSIZE)

		return 0;

	entry = MACHP(0)->pdb[PDX(va)];

	if(!(entry&PTEVALID))

		return 0;

	if(!(entry&PTESIZE)){

		/* make sure entry will help the fault */

		table = KADDR(PPN(entry));

		if(!(table[PTX(va)]&PTEVALID))

			return 0;

	}

	vpd[PDX(va)] = entry;

	/*

	 * TLB doesn't cache negative results, so no flush needed.

	 */

	return 1;

}

/*

 * KMap is used to map individual pages into virtual memory.

 * It is rare to have more than a few KMaps at a time (in the 

 * absence of interrupts, only two at a time are ever used,

 * but interrupts can stack).  The mappings are local to a process,

 * so we can use the same range of virtual address space for

 * all processes without any coordination.

 */

#define kpt (vpt+VPTX(KMAP))

#define NKPT (KMAPSIZE/BY2PG)

KMap*

kmap(Page *page)

{

	int i, o, s;

	if(up == nil)

		panic("kmap: up=0 pc=%#.8lux", getcallerpc(&page));

	if(up->mmupdb == nil)

		upallocpdb();

	if(up->nkmap < 0)

		panic("kmap %lud %s: nkmap=%d", up->pid, up->text, up->nkmap);

	/*

	 * Splhi shouldn't be necessary here, but paranoia reigns.

	 * See comment in putmmu above.

	 */

	s = splhi();

	up->nkmap++;

	if(!(vpd[PDX(KMAP)]&PTEVALID)){

		/* allocate page directory */

		if(KMAPSIZE > BY2XPG)

			panic("bad kmapsize");

		if(up->kmaptable != nil)

			panic("kmaptable");

		spllo();

		up->kmaptable = newpage(0, 0, 0);

		splhi();

		vpd[PDX(KMAP)] = up->kmaptable->pa|PTEWRITE|PTEVALID;

		flushpg((ulong)kpt);

		memset(kpt, 0, BY2PG);

		kpt[0] = page->pa|PTEWRITE|PTEVALID;

		up->lastkmap = 0;

		splx(s);

		return (KMap*)KMAP;

	}

	if(up->kmaptable == nil)

		panic("no kmaptable");

	o = up->lastkmap+1;

	for(i=0; i<NKPT; i++){

		if(kpt[(i+o)%NKPT] == 0){

			o = (i+o)%NKPT;

			kpt[o] = page->pa|PTEWRITE|PTEVALID;

			up->lastkmap = o;