This is xnu-8019. See this file in:
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/*-
 * Copyright (c) 1982, 1986, 1990, 1993
 *	The Regents of the University of California.  All rights reserved.
 * (c) UNIX System Laboratories, Inc.
 * All or some portions of this file are derived from material licensed
 * to the University of California by American Telephone and Telegraph
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 * the permission of UNIX System Laboratories, Inc.
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 *    notice, this list of conditions and the following disclaimer in the
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 *
 *	from: @(#)kern_physio.c	8.1 (Berkeley) 6/10/93
 */
/*
 * HISTORY
 * 27-July-97  Umesh Vaishampayan  (umeshv@apple.com)
 *	Allow physio() to kernel space.
 */

#include <sys/param.h>
#include <sys/systm.h>
#include <sys/buf_internal.h>
#include <sys/conf.h>
#include <sys/proc_internal.h>
#include <sys/uio_internal.h>
#include <kern/assert.h>

int
physio( void (*f_strategy)(buf_t),
    buf_t bp,
    dev_t dev,
    int flags,
    u_int (*f_minphys)(buf_t),
    struct uio *uio,
    int blocksize)
{
	struct proc *p = current_proc();
	int error, i, buf_allocated, todo;
	size_t iosize;
	int orig_bflags = 0;
	int64_t done;

	error = 0;
	flags &= B_READ | B_WRITE;
	buf_allocated = 0;

	/*
	 * [check user read/write access to the data buffer]
	 *
	 * Check each iov one by one.  Note that we know if we're reading or
	 * writing, so we ignore the uio's rw parameter.  Also note that if
	 * we're doing a read, that's a *write* to user-space.
	 */
	for (i = 0; i < uio->uio_iovcnt; i++) {
		if (UIO_SEG_IS_USER_SPACE(uio->uio_segflg)) {
			user_addr_t base;
			user_size_t len;

			if (uio_getiov(uio, i, &base, &len) ||
			    !useracc(base,
			    len,
			    (flags == B_READ) ? B_WRITE : B_READ)) {
				return EFAULT;
			}
		}
	}
	/*
	 * Make sure we have a buffer, creating one if necessary.
	 */
	if (bp == NULL) {
		bp = buf_alloc((vnode_t)0);
		buf_allocated = 1;
	} else {
		orig_bflags = buf_flags(bp);
	}
	/*
	 * at this point we should have a buffer
	 * that is marked BL_BUSY... we either
	 * acquired it via buf_alloc, or it was
	 * passed into us... if it was passed
	 * in, it needs to already be owned by
	 * the caller (i.e. BL_BUSY is set)
	 */
	assert(bp->b_lflags & BL_BUSY);

	/*
	 * [set up the fixed part of the buffer for a transfer]
	 */
	bp->b_dev = dev;
	bp->b_proc = p;

	/*
	 * [mark the buffer busy for physical I/O]
	 * (i.e. set B_PHYS (because it's an I/O to user
	 * memory, and B_RAW, because B_RAW is to be
	 * "Set by physio for raw transfers.", in addition
	 * to the read/write flag.)
	 */
	buf_setflags(bp, B_PHYS | B_RAW);

	/*
	 * [while there is data to transfer and no I/O error]
	 * Note that I/O errors are handled with a 'goto' at the bottom
	 * of the 'while' loop.
	 */
	while (uio_resid(uio) > 0) {
		iosize = uio_curriovlen(uio);
		if (iosize > MAXPHYSIO_WIRED) {
			iosize = MAXPHYSIO_WIRED;
		}

		/*
		 * make sure we're set to issue a fresh I/O
		 * in the right direction
		 */
		buf_reset(bp, flags);

		/* [set up the buffer for a maximum-sized transfer] */
		buf_setblkno(bp, uio_offset(uio) / blocksize);
		assert(iosize <= UINT32_MAX);
		buf_setcount(bp, (uint32_t)iosize);
		buf_setdataptr(bp, (uintptr_t)CAST_DOWN(caddr_t, uio_curriovbase(uio)));

		/*
		 * [call f_minphys to bound the tranfer size]
		 * and remember the amount of data to transfer,
		 * for later comparison.
		 */
		(*f_minphys)(bp);
		todo = buf_count(bp);

		/*
		 * [lock the part of the user address space involved
		 *    in the transfer]
		 */

		if (UIO_SEG_IS_USER_SPACE(uio->uio_segflg)) {
			error = vslock(CAST_USER_ADDR_T(buf_dataptr(bp)),
			    (user_size_t)todo);
			if (error) {
				goto finished;
			}
		}

		/* [call f_strategy to start the transfer] */
		(*f_strategy)(bp);


		/* [wait for the transfer to complete] */
		error = (int)buf_biowait(bp);

		/*
		 * [unlock the part of the address space previously
		 *    locked]
		 */
		if (UIO_SEG_IS_USER_SPACE(uio->uio_segflg)) {
			vsunlock(CAST_USER_ADDR_T(buf_dataptr(bp)),
			    (user_size_t)todo,
			    (flags & B_READ));
		}

		/*
		 * [deduct the transfer size from the total number
		 *    of data to transfer]
		 */
		done = buf_count(bp) - buf_resid(bp);
		assert(0 <= done && done <= UINT32_MAX);
		uio_update(uio, (user_size_t)done);

		/*
		 * Now, check for an error.
		 * Also, handle weird end-of-disk semantics.
		 */
		if (error || done < todo) {
			goto finished;
		}
	}

finished:
	if (buf_allocated) {
		buf_free(bp);
	} else {
		buf_setflags(bp, orig_bflags);
	}

	return error;
}

/*
 * Leffler, et al., says on p. 231:
 * "The minphys() routine is called by physio() to adjust the
 * size of each I/O transfer before the latter is passed to
 * the strategy routine..."
 *
 * so, just adjust the buffer's count accounting to MAXPHYS here,
 * and return the new count;
 */
u_int
minphys(struct buf *bp)
{
	buf_setcount(bp, min(MAXPHYS, buf_count(bp)));
	return buf_count(bp);
}