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bufferuio_offset指向这个bufferoffest,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

uiomoveuiomove_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

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