FreeBSD kernel 笔记（4）——UIO

UIO相关的结构体和函数定义：

 #include <sys/types.h>
 #include <sys/uio.h>

 struct uio {
     struct  iovec *uio_iov;         /* scatter/gather list */
     int     uio_iovcnt;         /* length of scatter/gather list */
     off_t   uio_offset;         /* offset in target object */
     ssize_t uio_resid;          /* remaining bytes to copy */
     enum    uio_seg uio_segflg;     /* address space */
     enum    uio_rw uio_rw;      /* operation */
     struct  thread *uio_td;         /* owner */
 };

 int
 uiomove(void *buf, int howmuch, struct uio *uiop);

 int
 uiomove_nofault(void *buf, int howmuch, struct uio *uiop);

关于uio结构体需要注意的是：如果uio_iovcnt不为1，可以把uio_iov所指向的struct iovec看成一个连接起来的大buffer。uio_offset指向这个buffer的offest，而uio_resid表明还有多少字节需要copy。在执行read操作时，uio_offset表明已经填充的buffer大小，而uio_resid表明buffer剩余的空间。可以参考这个程序。

uiomove和uiomove_nofault本质上调用的都是uiomove_faultflag函数：

static int
uiomove_faultflag(void *cp, int n, struct uio *uio, int nofault)
{
    struct thread *td;
    struct iovec *iov;
    size_t cnt;
    int error, newflags, save;

    td = curthread;
    error = 0;

    KASSERT(uio->uio_rw == UIO_READ || uio->uio_rw == UIO_WRITE,
    ("uiomove: mode"));
    KASSERT(uio->uio_segflg != UIO_USERSPACE || uio->uio_td == td,
    ("uiomove proc"));
    if (!nofault)
        WITNESS_WARN(WARN_GIANTOK | WARN_SLEEPOK, NULL,
        "Calling uiomove()");

    /* XXX does it make a sense to set TDP_DEADLKTREAT for UIO_SYSSPACE ? */
    newflags = TDP_DEADLKTREAT;
    if (uio->uio_segflg == UIO_USERSPACE && nofault) {
        /*
         * Fail if a non-spurious page fault occurs.
         */
        newflags |= TDP_NOFAULTING | TDP_RESETSPUR;
    }
    save = curthread_pflags_set(newflags);

    while (n > 0 && uio->uio_resid) {
        iov = uio->uio_iov;
        cnt = iov->iov_len;
        if (cnt == 0) {
            uio->uio_iov++;
            uio->uio_iovcnt--;
            continue;
        }
        if (cnt > n)
            cnt = n;

        switch (uio->uio_segflg) {

        case UIO_USERSPACE:
            maybe_yield();
            if (uio->uio_rw == UIO_READ)
                error = copyout(cp, iov->iov_base, cnt);
            else
                error = copyin(iov->iov_base, cp, cnt);
            if (error)
                goto out;
            break;

        case UIO_SYSSPACE:
            if (uio->uio_rw == UIO_READ)
                bcopy(cp, iov->iov_base, cnt);
            else
                bcopy(iov->iov_base, cp, cnt);
            break;
        case UIO_NOCOPY:
            break;
        }
        iov->iov_base = (char *)iov->iov_base + cnt;
        iov->iov_len -= cnt;
        uio->uio_resid -= cnt;
        uio->uio_offset += cnt;
        cp = (char *)cp + cnt;
        n -= cnt;
    }
out:
    curthread_pflags_restore(save);
    return (error);
}

可以看到这个函数会对传入的uio结构体的内容进行修改。

关于uiomove_nofault()函数，参考如下定义：

The function uiomovenofault() requires that the buffer and I/O vectors be accessible without incurring a page fault. The source and destination addresses must be physically mapped for read and write access, respec- tively, and neither the source nor destination addresses may be pageable. Thus, the function uiomovenofault() can be called from contexts where acquiring virtual memory system locks or sleeping are prohibited.

参考资料：
UIO。