[v2,01/12] util/iov: introduce qemu_iovec_init_extended

Introduce new initialization API, to create requests with padding. Will
be used in the following patch. New API uses qemu_iovec_init_buf if
resulting io vector has only one element, to avoid extra allocations.
So, we need to update qemu_iovec_destroy to support destroying such
QIOVs.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/qemu/iov.h |   7 +++
 util/iov.c         | 112 +++++++++++++++++++++++++++++++++++++++++++--
 2 files changed, 114 insertions(+), 5 deletions(-)

04.06.2019 19:15, Vladimir Sementsov-Ogievskiy wrote:
> Introduce new initialization API, to create requests with padding. Will
> be used in the following patch. New API uses qemu_iovec_init_buf if
> resulting io vector has only one element, to avoid extra allocations.
> So, we need to update qemu_iovec_destroy to support destroying such
> QIOVs.
> 
> Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
> ---
>   include/qemu/iov.h |   7 +++
>   util/iov.c         | 112 +++++++++++++++++++++++++++++++++++++++++++--
>   2 files changed, 114 insertions(+), 5 deletions(-)
> 
> diff --git a/include/qemu/iov.h b/include/qemu/iov.h
> index 48b45987b7..f3787a0cf7 100644
> --- a/include/qemu/iov.h
> +++ b/include/qemu/iov.h
> @@ -199,6 +199,13 @@ static inline void *qemu_iovec_buf(QEMUIOVector *qiov)
>   
>   void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint);
>   void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov);
> +void qemu_iovec_init_extended(
> +        QEMUIOVector *qiov,
> +        void *head_buf, size_t head_len,
> +        QEMUIOVector *mid_qiov, size_t mid_offset, size_t mid_len,
> +        void *tail_buf, size_t tail_len);
> +void qemu_iovec_init_slice(QEMUIOVector *qiov, QEMUIOVector *source,
> +                           size_t offset, size_t len);
>   void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len);
>   void qemu_iovec_concat(QEMUIOVector *dst,
>                          QEMUIOVector *src, size_t soffset, size_t sbytes);
> diff --git a/util/iov.c b/util/iov.c
> index 74e6ca8ed7..39b6e31494 100644
> --- a/util/iov.c
> +++ b/util/iov.c
> @@ -353,6 +353,103 @@ void qemu_iovec_concat(QEMUIOVector *dst,
>       qemu_iovec_concat_iov(dst, src->iov, src->niov, soffset, sbytes);
>   }
>   
> +/*
> + * qiov_find_iov
> + *
> + * Return pointer to iovec structure, where byte at @offset in original vector
> + * @iov exactly is.
> + * Set @remaining_offset to be offset inside that iovec to the same byte.
> + */
> +static struct iovec *iov_skip_offset(struct iovec *iov, size_t offset,
> +                                     size_t *remaining_offset)
> +{
> +    while (offset > 0 && offset >= iov->iov_len) {
> +        offset -= iov->iov_len;
> +        iov++;
> +    }
> +    *remaining_offset = offset;
> +
> +    return iov;
> +}
> +
> +/*
> + * qiov_slice
> + *
> + * Find subarray of iovec's, containing requested range. @head would
> + * be offset in first iov (returned by the function), @tail would be
> + * count of extra bytes in last iovec (returned iov + @niov - 1).
> + */
> +static struct iovec *qiov_slice(QEMUIOVector *qiov,
> +                                size_t offset, size_t len,
> +                                size_t *head, size_t *tail, int *niov)
> +{
> +    struct iovec *iov, *end_iov;
> +
> +    assert(offset + len <= qiov->size);
> +
> +    iov = iov_skip_offset(qiov->iov, offset, head);
> +    end_iov = iov_skip_offset(iov, *head + len, tail);
> +
> +    if (*tail > 0) {
> +        assert(*tail < end_iov->iov_len);
> +        *tail = end_iov->iov_len - *tail;
> +        end_iov++;
> +    }
> +
> +    *niov = end_iov - iov;
> +
> +    return iov;
> +}
> +
> +/*
> + * Compile new iovec, combining @head_buf buffer, sub-qiov of @mid_qiov,
> + * and @tail_buf buffer into new qiov.
> + */
> +void qemu_iovec_init_extended(
> +        QEMUIOVector *qiov,
> +        void *head_buf, size_t head_len,
> +        QEMUIOVector *mid_qiov, size_t mid_offset, size_t mid_len,
> +        void *tail_buf, size_t tail_len)
> +{
> +    size_t mid_head, mid_tail;
> +    int total_niov, mid_niov;

Oops, clang is right, mid_niov may be uninitialized. So, here should be "mid_niov = 0".

> +    struct iovec *p, *mid_iov;
> +
> +    if (mid_len) {
> +        mid_iov = qiov_slice(mid_qiov, mid_offset, mid_len,
> +                             &mid_head, &mid_tail, &mid_niov);
> +    }
> +
> +    total_niov = !!head_len + mid_niov + !!tail_len;
> +    if (total_niov == 1) {
> +        qemu_iovec_init_buf(qiov, NULL, 0);
> +        p = &qiov->local_iov;
> +    } else {
> +        qiov->niov = qiov->nalloc = total_niov;
> +        qiov->size = head_len + mid_len + tail_len;
> +        p = qiov->iov = g_new(struct iovec, qiov->niov);
> +    }
> +
> +    if (head_len) {
> +        p->iov_base = head_buf;
> +        p->iov_len = head_len;
> +        p++;
> +    }
> +
> +    if (mid_len) {
> +        memcpy(p, mid_iov, mid_niov * sizeof(*p));
> +        p[0].iov_base = (uint8_t *)p[0].iov_base + mid_head;
> +        p[0].iov_len -= mid_head;
> +        p[mid_niov - 1].iov_len -= mid_tail;
> +        p += mid_niov;
> +    }
> +
> +    if (tail_len) {
> +        p->iov_base = tail_buf;
> +        p->iov_len = tail_len;
> +    }
> +}
> +
>   /*
>    * Check if the contents of the iovecs are all zero
>    */
> @@ -374,14 +471,19 @@ bool qemu_iovec_is_zero(QEMUIOVector *qiov)
>       return true;
>   }
>   
> +void qemu_iovec_init_slice(QEMUIOVector *qiov, QEMUIOVector *source,
> +                           size_t offset, size_t len)
> +{
> +    qemu_iovec_init_extended(qiov, NULL, 0, source, offset, len, NULL, 0);
> +}
> +
>   void qemu_iovec_destroy(QEMUIOVector *qiov)
>   {
> -    assert(qiov->nalloc != -1);
> +    if (qiov->nalloc != -1) {
> +        g_free(qiov->iov);
> +    }
>   
> -    qemu_iovec_reset(qiov);
> -    g_free(qiov->iov);
> -    qiov->nalloc = 0;
> -    qiov->iov = NULL;
> +    memset(qiov, 0, sizeof(*qiov));
>   }
>   
>   void qemu_iovec_reset(QEMUIOVector *qiov)
>