xref: /freebsd/sys/i386/i386/vm_machdep.c (revision 24a1cce34fc10ade769c2d2f2e00ca5ad820a671)

/*-
 * Copyright (c) 1982, 1986 The Regents of the University of California.
 * Copyright (c) 1989, 1990 William Jolitz
 * Copyright (c) 1994 John Dyson
 * All rights reserved.
 *
 * This code is derived from software contributed to Berkeley by
 * the Systems Programming Group of the University of Utah Computer
 * Science Department, and William Jolitz.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 * 1. Redistributions of source code must retain the above copyright
 *    notice, this list of conditions and the following disclaimer.
 * 2. Redistributions in binary form must reproduce the above copyright
 *    notice, this list of conditions and the following disclaimer in the
 *    documentation and/or other materials provided with the distribution.
 * 3. All advertising materials mentioning features or use of this software
 *    must display the following acknowledgement:
 *	This product includes software developed by the University of
 *	California, Berkeley and its contributors.
 * 4. Neither the name of the University nor the names of its contributors
 *    may be used to endorse or promote products derived from this software
 *    without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
 * ARE DISCLAIMED.  IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
 * SUCH DAMAGE.
 *
 *	from: @(#)vm_machdep.c	7.3 (Berkeley) 5/13/91
 *	Utah $Hdr: vm_machdep.c 1.16.1.1 89/06/23$
 *	$Id: vm_machdep.c,v 1.39 1995/05/30 07:59:46 rgrimes Exp $
 */

#include "npx.h"
#include <sys/param.h>
#include <sys/systm.h>
#include <sys/proc.h>
#include <sys/malloc.h>
#include <sys/buf.h>
#include <sys/vnode.h>
#include <sys/user.h>

#include <machine/clock.h>
#include <machine/cpu.h>
#include <machine/md_var.h>

#include <vm/vm.h>
#include <vm/vm_kern.h>
#include <vm/vm_page.h>

#include <i386/isa/isa.h>

#ifdef BOUNCE_BUFFERS
vm_map_t	io_map;
volatile int	kvasfreecnt;


caddr_t		bouncememory;
int		bouncepages, bpwait;
vm_offset_t	*bouncepa;
int		bmwait, bmfreeing;

#define BITS_IN_UNSIGNED (8*sizeof(unsigned))
int		bounceallocarraysize;
unsigned	*bounceallocarray;
int		bouncefree;

#define SIXTEENMEG (4096*4096)
#define MAXBKVA 1024
int		maxbkva = MAXBKVA*NBPG;

/* special list that can be used at interrupt time for eventual kva free */
struct kvasfree {
	vm_offset_t addr;
	vm_offset_t size;
} kvaf[MAXBKVA];


vm_offset_t vm_bounce_kva();
/*
 * get bounce buffer pages (count physically contiguous)
 * (only 1 inplemented now)
 */
vm_offset_t
vm_bounce_page_find(count)
	int count;
{
	int bit;
	int s,i;

	if (count != 1)
		panic("vm_bounce_page_find -- no support for > 1 page yet!!!");

	s = splbio();
retry:
	for (i = 0; i < bounceallocarraysize; i++) {
		if (bounceallocarray[i] != 0xffffffff) {
			bit = ffs(~bounceallocarray[i]);
			if (bit) {
				bounceallocarray[i] |= 1 << (bit - 1) ;
				bouncefree -= count;
				splx(s);
				return bouncepa[(i * BITS_IN_UNSIGNED + (bit - 1))];
			}
		}
	}
	bpwait = 1;
	tsleep((caddr_t) &bounceallocarray, PRIBIO, "bncwai", 0);
	goto retry;
}

void
vm_bounce_kva_free(addr, size, now)
	vm_offset_t addr;
	vm_offset_t size;
	int now;
{
	int s = splbio();
	kvaf[kvasfreecnt].addr = addr;
	kvaf[kvasfreecnt].size = size;
	++kvasfreecnt;
	if( now) {
		/*
		 * this will do wakeups
		 */
		vm_bounce_kva(0,0);
	} else {
		if (bmwait) {
		/*
		 * if anyone is waiting on the bounce-map, then wakeup
		 */
			wakeup((caddr_t) io_map);
			bmwait = 0;
		}
	}
	splx(s);
}

/*
 * free count bounce buffer pages
 */
void
vm_bounce_page_free(pa, count)
	vm_offset_t pa;
	int count;
{
	int allocindex;
	int index;
	int bit;

	if (count != 1)
		panic("vm_bounce_page_free -- no support for > 1 page yet!!!");

	for(index=0;index<bouncepages;index++) {
		if( pa == bouncepa[index])
			break;
	}

	if( index == bouncepages)
		panic("vm_bounce_page_free: invalid bounce buffer");

	allocindex = index / BITS_IN_UNSIGNED;
	bit = index % BITS_IN_UNSIGNED;

	bounceallocarray[allocindex] &= ~(1 << bit);

	bouncefree += count;
	if (bpwait) {
		bpwait = 0;
		wakeup((caddr_t) &bounceallocarray);
	}
}

/*
 * allocate count bounce buffer kva pages
 */
vm_offset_t
vm_bounce_kva(size, waitok)
	int size;
	int waitok;
{
	int i;
	vm_offset_t kva = 0;
	vm_offset_t off;
	int s = splbio();
more:
	if (!bmfreeing && kvasfreecnt) {
		bmfreeing = 1;
		for (i = 0; i < kvasfreecnt; i++) {
			for(off=0;off<kvaf[i].size;off+=NBPG) {
				pmap_kremove( kvaf[i].addr + off);
			}
			kmem_free_wakeup(io_map, kvaf[i].addr,
				kvaf[i].size);
		}
		kvasfreecnt = 0;
		bmfreeing = 0;
		if( bmwait) {
			bmwait = 0;
			wakeup( (caddr_t) io_map);
		}
	}

	if( size == 0) {
		splx(s);
		return NULL;
	}

	if ((kva = kmem_alloc_pageable(io_map, size)) == 0) {
		if( !waitok) {
			splx(s);
			return NULL;
		}
		bmwait = 1;
		tsleep((caddr_t) io_map, PRIBIO, "bmwait", 0);
		goto more;
	}
	splx(s);
	return kva;
}

/*
 * same as vm_bounce_kva -- but really allocate (but takes pages as arg)
 */
vm_offset_t
vm_bounce_kva_alloc(count)
int count;
{
	int i;
	vm_offset_t kva;
	vm_offset_t pa;
	if( bouncepages == 0) {
		kva = (vm_offset_t) malloc(count*NBPG, M_TEMP, M_WAITOK);
		return kva;
	}
	kva = vm_bounce_kva(count*NBPG, 1);
	for(i=0;i<count;i++) {
		pa = vm_bounce_page_find(1);
		pmap_kenter(kva + i * NBPG, pa);
	}
	return kva;
}

/*
 * same as vm_bounce_kva_free -- but really free
 */
void
vm_bounce_kva_alloc_free(kva, count)
	vm_offset_t kva;
	int count;
{
	int i;
	vm_offset_t pa;
	if( bouncepages == 0) {
		free((caddr_t) kva, M_TEMP);
		return;
	}
	for(i = 0; i < count; i++) {
		pa = pmap_kextract(kva + i * NBPG);
		vm_bounce_page_free(pa, 1);
	}
	vm_bounce_kva_free(kva, count*NBPG, 0);
}

/*
 * do the things necessary to the struct buf to implement
 * bounce buffers...  inserted before the disk sort
 */
void
vm_bounce_alloc(bp)
	struct buf *bp;
{
	int countvmpg;
	vm_offset_t vastart, vaend;
	vm_offset_t vapstart, vapend;
	vm_offset_t va, kva;
	vm_offset_t pa;
	int dobounceflag = 0;
	int i;

	if (bouncepages == 0)
		return;

	if (bp->b_flags & B_BOUNCE) {
		printf("vm_bounce_alloc: called recursively???\n");
		return;
	}

	if (bp->b_bufsize < bp->b_bcount) {
		printf(
		    "vm_bounce_alloc: b_bufsize(0x%lx) < b_bcount(0x%lx) !!\n",
			bp->b_bufsize, bp->b_bcount);
		panic("vm_bounce_alloc");
	}

/*
 *  This is not really necessary
 *	if( bp->b_bufsize != bp->b_bcount) {
 *		printf("size: %d, count: %d\n", bp->b_bufsize, bp->b_bcount);
 *	}
 */


	vastart = (vm_offset_t) bp->b_data;
	vaend = (vm_offset_t) bp->b_data + bp->b_bufsize;

	vapstart = i386_trunc_page(vastart);
	vapend = i386_round_page(vaend);
	countvmpg = (vapend - vapstart) / NBPG;

/*
 * if any page is above 16MB, then go into bounce-buffer mode
 */
	va = vapstart;
	for (i = 0; i < countvmpg; i++) {
		pa = pmap_kextract(va);
		if (pa >= SIXTEENMEG)
			++dobounceflag;
		if( pa == 0)
			panic("vm_bounce_alloc: Unmapped page");
		va += NBPG;
	}
	if (dobounceflag == 0)
		return;

	if (bouncepages < dobounceflag)
		panic("Not enough bounce buffers!!!");

/*
 * allocate a replacement kva for b_addr
 */
	kva = vm_bounce_kva(countvmpg*NBPG, 1);
#if 0
	printf("%s: vapstart: %x, vapend: %x, countvmpg: %d, kva: %x ",
		(bp->b_flags & B_READ) ? "read":"write",
			vapstart, vapend, countvmpg, kva);
#endif
	va = vapstart;
	for (i = 0; i < countvmpg; i++) {
		pa = pmap_kextract(va);
		if (pa >= SIXTEENMEG) {
			/*
			 * allocate a replacement page
			 */
			vm_offset_t bpa = vm_bounce_page_find(1);
			pmap_kenter(kva + (NBPG * i), bpa);
#if 0
			printf("r(%d): (%x,%x,%x) ", i, va, pa, bpa);
#endif
			/*
			 * if we are writing, the copy the data into the page
			 */
			if ((bp->b_flags & B_READ) == 0) {
				bcopy((caddr_t) va, (caddr_t) kva + (NBPG * i), NBPG);
			}
		} else {
			/*
			 * use original page
			 */
			pmap_kenter(kva + (NBPG * i), pa);
		}
		va += NBPG;
	}

/*
 * flag the buffer as being bounced
 */
	bp->b_flags |= B_BOUNCE;
/*
 * save the original buffer kva
 */
	bp->b_savekva = bp->b_data;
/*
 * put our new kva into the buffer (offset by original offset)
 */
	bp->b_data = (caddr_t) (((vm_offset_t) kva) |
				((vm_offset_t) bp->b_savekva & (NBPG - 1)));
#if 0
	printf("b_savekva: %x, newva: %x\n", bp->b_savekva, bp->b_data);
#endif
	return;
}

/*
 * hook into biodone to free bounce buffer
 */
void
vm_bounce_free(bp)
	struct buf *bp;
{
	int i;
	vm_offset_t origkva, bouncekva, bouncekvaend;

/*
 * if this isn't a bounced buffer, then just return
 */
	if ((bp->b_flags & B_BOUNCE) == 0)
		return;

/*
 *  This check is not necessary
 *	if (bp->b_bufsize != bp->b_bcount) {
 *		printf("vm_bounce_free: b_bufsize=%d, b_bcount=%d\n",
 *			bp->b_bufsize, bp->b_bcount);
 *	}
 */

	origkva = (vm_offset_t) bp->b_savekva;
	bouncekva = (vm_offset_t) bp->b_data;
/*
	printf("free: %d ", bp->b_bufsize);
*/

/*
 * check every page in the kva space for b_addr
 */
	for (i = 0; i < bp->b_bufsize; ) {
		vm_offset_t mybouncepa;
		vm_offset_t copycount;

		copycount = i386_round_page(bouncekva + 1) - bouncekva;
		mybouncepa = pmap_kextract(i386_trunc_page(bouncekva));

/*
 * if this is a bounced pa, then process as one
 */
		if ( mybouncepa != pmap_kextract( i386_trunc_page( origkva))) {
			vm_offset_t tocopy = copycount;
			if (i + tocopy > bp->b_bufsize)
				tocopy = bp->b_bufsize - i;
/*
 * if this is a read, then copy from bounce buffer into original buffer
 */
			if (bp->b_flags & B_READ)
				bcopy((caddr_t) bouncekva, (caddr_t) origkva, tocopy);
/*
 * free the bounce allocation
 */

/*
			printf("(kva: %x, pa: %x)", bouncekva, mybouncepa);
*/
			vm_bounce_page_free(mybouncepa, 1);
		}

		origkva += copycount;
		bouncekva += copycount;
		i += copycount;
	}

/*
	printf("\n");
*/
/*
 * add the old kva into the "to free" list
 */

	bouncekva= i386_trunc_page((vm_offset_t) bp->b_data);
	bouncekvaend= i386_round_page((vm_offset_t)bp->b_data + bp->b_bufsize);

/*
	printf("freeva: %d\n", (bouncekvaend - bouncekva) / NBPG);
*/
	vm_bounce_kva_free( bouncekva, (bouncekvaend - bouncekva), 0);
	bp->b_data = bp->b_savekva;
	bp->b_savekva = 0;
	bp->b_flags &= ~B_BOUNCE;

	return;
}


/*
 * init the bounce buffer system
 */
void
vm_bounce_init()
{
	int i;

	kvasfreecnt = 0;

	if (bouncepages == 0)
		return;

	bounceallocarraysize = (bouncepages + BITS_IN_UNSIGNED - 1) / BITS_IN_UNSIGNED;
	bounceallocarray = malloc(bounceallocarraysize * sizeof(unsigned), M_TEMP, M_NOWAIT);

	if (!bounceallocarray)
		panic("Cannot allocate bounce resource array");

	bouncepa = malloc(bouncepages * sizeof(vm_offset_t), M_TEMP, M_NOWAIT);
	if (!bouncepa)
		panic("Cannot allocate physical memory array");

	for(i=0;i<bounceallocarraysize;i++) {
		bounceallocarray[i] = 0xffffffff;
	}

	for(i=0;i<bouncepages;i++) {
		vm_offset_t pa;
		if( (pa = pmap_kextract((vm_offset_t) bouncememory + i * NBPG)) >= SIXTEENMEG)
			panic("bounce memory out of range");
		if( pa == 0)
			panic("bounce memory not resident");
		bouncepa[i] = pa;
		bounceallocarray[i/(8*sizeof(int))] &= ~(1<<(i%(8*sizeof(int))));
	}
	bouncefree = bouncepages;

}
#endif /* BOUNCE_BUFFERS */
/*
 * quick version of vm_fault
 */

void
vm_fault_quick( v, prot)
	vm_offset_t v;
	int prot;
{
	if (prot & VM_PROT_WRITE)
		subyte((char *)v, fubyte((char *)v));
	else
		(void) fubyte((char *)v);
}


/*
 * Finish a fork operation, with process p2 nearly set up.
 * Copy and update the kernel stack and pcb, making the child
 * ready to run, and marking it so that it can return differently
 * than the parent.  Returns 1 in the child process, 0 in the parent.
 * We currently double-map the user area so that the stack is at the same
 * address in each process; in the future we will probably relocate
 * the frame pointers on the stack after copying.
 */
int
cpu_fork(p1, p2)
	register struct proc *p1, *p2;
{
	register struct user *up = p2->p_addr;
	int offset;

	/*
	 * Copy pcb and stack from proc p1 to p2.
	 * We do this as cheaply as possible, copying only the active
	 * part of the stack.  The stack and pcb need to agree;
	 * this is tricky, as the final pcb is constructed by savectx,
	 * but its frame isn't yet on the stack when the stack is copied.
	 * swtch compensates for this when the child eventually runs.
	 * This should be done differently, with a single call
	 * that copies and updates the pcb+stack,
	 * replacing the bcopy and savectx.
	 */
	p2->p_addr->u_pcb = p1->p_addr->u_pcb;
	offset = mvesp() - (int)kstack;
	bcopy((caddr_t)kstack + offset, (caddr_t)p2->p_addr + offset,
	    (unsigned) ctob(UPAGES) - offset);
	p2->p_md.md_regs = p1->p_md.md_regs;

	pmap_activate(&p2->p_vmspace->vm_pmap, &up->u_pcb);

	/*
	 *
	 * Arrange for a non-local goto when the new process
	 * is started, to resume here, returning nonzero from setjmp.
	 */
	if (savectx(&up->u_pcb, 1)) {
		/*
		 * Return 1 in child.
		 */
		return (1);
	}
	return (0);
}

void
cpu_exit(p)
	register struct proc *p;
{

#if NNPX > 0
	npxexit(p);
#endif	/* NNPX */
	cnt.v_swtch++;
	cpu_switch(p);
	panic("cpu_exit");
}

void
cpu_wait(p) struct proc *p; {
/*	extern vm_map_t upages_map; */

	/* drop per-process resources */
 	pmap_remove(vm_map_pmap(u_map), (vm_offset_t) p->p_addr,
		((vm_offset_t) p->p_addr) + ctob(UPAGES));
	kmem_free(u_map, (vm_offset_t)p->p_addr, ctob(UPAGES));
	vmspace_free(p->p_vmspace);
}

/*
 * Dump the machine specific header information at the start of a core dump.
 */
int
cpu_coredump(p, vp, cred)
	struct proc *p;
	struct vnode *vp;
	struct ucred *cred;
{

	return (vn_rdwr(UIO_WRITE, vp, (caddr_t) p->p_addr, ctob(UPAGES),
	    (off_t)0, UIO_SYSSPACE, IO_NODELOCKED|IO_UNIT, cred, (int *)NULL,
	    p));
}

/*
 * Set a red zone in the kernel stack after the u. area.
 */
void
setredzone(pte, vaddr)
	u_short *pte;
	caddr_t vaddr;
{
/* eventually do this by setting up an expand-down stack segment
   for ss0: selector, allowing stack access down to top of u.
   this means though that protection violations need to be handled
   thru a double fault exception that must do an integral task
   switch to a known good context, within which a dump can be
   taken. a sensible scheme might be to save the initial context
   used by sched (that has physical memory mapped 1:1 at bottom)
   and take the dump while still in mapped mode */
}

/*
 * Move pages from one kernel virtual address to another.
 * Both addresses are assumed to reside in the Sysmap,
 * and size must be a multiple of CLSIZE.
 */

void
pagemove(from, to, size)
	register caddr_t from, to;
	int size;
{
	register vm_offset_t pa;

	if (size & CLOFSET)
		panic("pagemove");
	while (size > 0) {
		pa = pmap_kextract((vm_offset_t)from);
		if (pa == 0)
			panic("pagemove 2");
		if (pmap_kextract((vm_offset_t)to) != 0)
			panic("pagemove 3");
		pmap_kremove((vm_offset_t)from);
		pmap_kenter((vm_offset_t)to, pa);
		from += PAGE_SIZE;
		to += PAGE_SIZE;
		size -= PAGE_SIZE;
	}
}

/*
 * Convert kernel VA to physical address
 */
u_long
kvtop(void *addr)
{
	vm_offset_t va;

	va = pmap_kextract((vm_offset_t)addr);
	if (va == 0)
		panic("kvtop: zero page frame");
	return((int)va);
}

/*
 * Map an IO request into kernel virtual address space.
 *
 * All requests are (re)mapped into kernel VA space.
 * Notice that we use b_bufsize for the size of the buffer
 * to be mapped.  b_bcount might be modified by the driver.
 */
void
vmapbuf(bp)
	register struct buf *bp;
{
	register int npf;
	register caddr_t addr;
	int off;
	vm_offset_t kva;
	vm_offset_t pa, v;

	if ((bp->b_flags & B_PHYS) == 0)
		panic("vmapbuf");

	/*
	 * this is the kva that is to be used for
	 * the temporary kernel mapping
	 */
	kva = (vm_offset_t) bp->b_saveaddr;

	for (addr = (caddr_t)trunc_page(bp->b_data);
		addr < bp->b_data + bp->b_bufsize;
		addr += PAGE_SIZE) {

/*
 * do the vm_fault if needed, do the copy-on-write thing when
 * reading stuff off device into memory.
 */
		vm_fault_quick(addr,
			(bp->b_flags&B_READ)?(VM_PROT_READ|VM_PROT_WRITE):VM_PROT_READ);
		pa = pmap_kextract((vm_offset_t) addr);
		if (pa == 0)
			panic("vmapbuf: page not present");
/*
 * hold the data page
 */
#ifdef DIAGNOSTIC
		if( VM_PAGE_TO_PHYS(PHYS_TO_VM_PAGE(pa)) != pa)
			panic("vmapbuf: confused PHYS_TO_VM_PAGE mapping");
#endif
		vm_page_hold(PHYS_TO_VM_PAGE(pa));
	}

	addr = bp->b_saveaddr = bp->b_data;
	off = (int)addr & PGOFSET;
	npf = btoc(round_page(bp->b_bufsize + off));
	bp->b_data = (caddr_t) (kva + off);
	while (npf--) {
		pa = pmap_kextract((vm_offset_t)addr);
		if (pa == 0)
			panic("vmapbuf: null page frame");
		pmap_kenter(kva, trunc_page(pa));
		addr += PAGE_SIZE;
		kva += PAGE_SIZE;
	}
}

/*
 * Free the io map PTEs associated with this IO operation.
 * We also invalidate the TLB entries and restore the original b_addr.
 */
void
vunmapbuf(bp)
	register struct buf *bp;
{
	register caddr_t addr;
	vm_offset_t v,pa;

	if ((bp->b_flags & B_PHYS) == 0)
		panic("vunmapbuf");

	for (addr = (caddr_t)trunc_page((vm_offset_t) bp->b_data);
		addr < bp->b_data + bp->b_bufsize;
		addr += NBPG)
		pmap_kremove((vm_offset_t) addr);

	bp->b_data = bp->b_saveaddr;
	bp->b_saveaddr = NULL;

/*
 * unhold the pde, and data pages
 */
	for (addr = (caddr_t)trunc_page((vm_offset_t) bp->b_data);
		addr < bp->b_data + bp->b_bufsize;
		addr += NBPG) {
	/*
	 * release the data page
	 */
		pa = pmap_kextract((vm_offset_t) addr);
		vm_page_unhold(PHYS_TO_VM_PAGE(pa));
	}
}

/*
 * Force reset the processor by invalidating the entire address space!
 */
void
cpu_reset() {

	/*
	 * Attempt to do a CPU reset via the keyboard controller,
	 * do not turn of the GateA20, as any machine that fails
	 * to do the reset here would then end up in no man's land.
	 */

#ifndef BROKEN_KEYBOARD_RESET
	outb(IO_KBD + 4, 0xFE);
	DELAY(500000);	/* wait 0.5 sec to see if that did it */
	printf("Keyboard reset did not work, attempting CPU shutdown\n");
	DELAY(1000000);	/* wait 1 sec for printf to complete */
#endif

	/* force a shutdown by unmapping entire address space ! */
	bzero((caddr_t) PTD, NBPG);

	/* "good night, sweet prince .... <THUNK!>" */
	pmap_update();
	/* NOTREACHED */
	while(1);
}

/*
 * Grow the user stack to allow for 'sp'. This version grows the stack in
 *	chunks of SGROWSIZ.
 */
int
grow(p, sp)
	struct proc *p;
	u_int sp;
{
	unsigned int nss;
	caddr_t v;
	struct vmspace *vm = p->p_vmspace;

	if ((caddr_t)sp <= vm->vm_maxsaddr || (unsigned)sp >= (unsigned)USRSTACK)
	    return (1);

	nss = roundup(USRSTACK - (unsigned)sp, PAGE_SIZE);

	if (nss > p->p_rlimit[RLIMIT_STACK].rlim_cur)
		return (0);

	if (vm->vm_ssize && roundup(vm->vm_ssize << PAGE_SHIFT,
	    SGROWSIZ) < nss) {
		int grow_amount;
		/*
		 * If necessary, grow the VM that the stack occupies
		 * to allow for the rlimit. This allows us to not have
		 * to allocate all of the VM up-front in execve (which
		 * is expensive).
		 * Grow the VM by the amount requested rounded up to
		 * the nearest SGROWSIZ to provide for some hysteresis.
		 */
		grow_amount = roundup((nss - (vm->vm_ssize << PAGE_SHIFT)), SGROWSIZ);
		v = (char *)USRSTACK - roundup(vm->vm_ssize << PAGE_SHIFT,
		    SGROWSIZ) - grow_amount;
		/*
		 * If there isn't enough room to extend by SGROWSIZ, then
		 * just extend to the maximum size
		 */
		if (v < vm->vm_maxsaddr) {
			v = vm->vm_maxsaddr;
			grow_amount = MAXSSIZ - (vm->vm_ssize << PAGE_SHIFT);
		}
		if ((grow_amount == 0) || (vm_map_find(&vm->vm_map, NULL, 0, (vm_offset_t *)&v,
		    grow_amount, FALSE) != KERN_SUCCESS)) {
			return (0);
		}
		vm->vm_ssize += grow_amount >> PAGE_SHIFT;
	}

	return (1);
}