os161中的UIO_USERISPACE，UIO_USERSPACE和UIO_SYSSPACE是什么？

问题描述

我正在尝试完成操作系统课程Here的作业。

我对作业有疑问：

UIO_USERISPACE和UIO_USERSPACE有什么区别？什么时候应该使用UIO_SYSSPACE代替？

当我在源代码中搜索here时，我发现了这一点：

/* Source/destination. */
enum uio_seg {
        UIO_USERISPACE,/* User process code. */
        UIO_USERSPACE,/* User process data. */
        UIO_SYSSPACE,/* Kernel. */
};

struct uio {
    struct iovec     *uio_iov;  /* Data blocks */
    unsigned          uio_iovcnt;   /* Number of iovecs */
    off_t             uio_offset;   /* Desired offset into object */
    size_t            uio_resid;    /* Remaining amt of data to xfer */
    enum uio_seg      uio_segflg;   /* What kind of pointer we have */
    enum uio_rw       uio_rw;   /* Whether op is a read or write */
    struct addrspace *uio_space;    /* Address space for user pointer */
};

我可以看到它在结构uio中使用（我将在另一个问题中对此进行询问）以指示“使用了哪种指针”。

此外，我可以在函数uiomove，here的描述中看到它：

 *   (5) if uio_seg is UIO_SYSSPACE,set uio_space to NULL; otherwise,*       initialize uio_space to the address space in which the buffer
 *       should be found.

函数uiomove，here的完整说明：

/*
 * Copy data from a kernel buffer to a data region defined by a uio struct,* updating the uio struct's offset and resid fields. May alter the iovec
 * fields as well.
 *
 * Before calling this,you should
 *   (1) set up uio_iov to point to the buffer(s) you want to transfer
 *       to,and set uio_iovcnt to the number of such buffers;
 *   (2) initialize uio_offset as desired;
 *   (3) initialize uio_resid to the total amount of data that can be
 *       transferred through this uio;
 *   (4) set up uio_seg and uio_rw correctly;
 *   (5) if uio_seg is UIO_SYSSPACE,*       initialize uio_space to the address space in which the buffer
 *       should be found.
 *
 * After calling,*   (1) the contents of uio_iov and uio_iovcnt may be altered and
 *       should not be interpreted;
 *   (2) uio_offset will have been incremented by the amount transferred;
 *   (3) uio_resid will have been decremented by the amount transferred;
 *   (4) uio_segflg,uio_rw,and uio_space will be unchanged.
 *
 * uiomove() may be called repeatedly on the same uio to transfer
 * additional data until the available buffer space the uio refers to
 * is exhausted.
 *
 * Note that the actual value of uio_offset is not interpreted. It is
 * provided (and updated by uiomove) to allow for easier file seek
 * pointer management.
 *
 * When uiomove is called,the address space presently in context must
 * be the same as the one recorded in uio_space. This is an important
 * sanity check if I/O has been queued.
 */

函数uiomove，here的定义：

int
uiomove(void *ptr,size_t n,struct uio *uio)
{
    struct iovec *iov;
    size_t size;
    int result;

    if (uio->uio_rw != UIO_READ && uio->uio_rw != UIO_WRITE) {
        panic("uiomove: Invalid uio_rw %d\n",(int) uio->uio_rw);
    }
    if (uio->uio_segflg==UIO_SYSSPACE) {
        KASSERT(uio->uio_space == NULL);
    }
    else {
        KASSERT(uio->uio_space == proc_getas());
    }

    while (n > 0 && uio->uio_resid > 0) {
        /* get the first iovec */
        iov = uio->uio_iov;
        size = iov->iov_len;

        if (size > n) {
            size = n;
        }

        if (size == 0) {
            /* move to the next iovec and try again */
            uio->uio_iov++;
            uio->uio_iovcnt--;
            if (uio->uio_iovcnt == 0) {
                /*
                 * This should only happen if you set
                 * uio_resid incorrectly (to more than
                 * the total length of buffers the uio
                 * points to).
                 */
                panic("uiomove: ran out of buffers\n");
            }
            continue;
        }

        switch (uio->uio_segflg) {
            case UIO_SYSSPACE:
                if (uio->uio_rw == UIO_READ) {
                    memmove(iov->iov_kbase,ptr,size);
                }
                else {
                    memmove(ptr,iov->iov_kbase,size);
                }
                iov->iov_kbase = ((char *)iov->iov_kbase+size);
                break;
            case UIO_USERSPACE:
            case UIO_USERISPACE:
                if (uio->uio_rw == UIO_READ) {
                    result = copyout(ptr,iov->iov_ubase,size);
                }
                else {
                    result = copyin(iov->iov_ubase,size);
                }
                if (result) {
                    return result;
                }
                iov->iov_ubase += size;
                break;
            default:
                panic("uiomove: Invalid uio_segflg %d\n",(int)uio->uio_segflg);
        }

        iov->iov_len -= size;
        uio->uio_resid -= size;
        uio->uio_offset += size;
        ptr = ((char *)ptr + size);
        n -= size;
    }

    return 0;
}

我似乎不赞成使用它。谁能给我一些关于他们的直觉？例如，何时何地使用它们？他们的目的是什么？而且，对于原始问题的答案，UIO_USERISPACE和UIO_USERSPACE有什么区别？什么时候应该使用UIO_SYSSPACE代替？

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