util/iov: Make qiov_slice() public

We want to inline qemu_iovec_init_extended() in block/io.c for padding
requests, and having access to qiov_slice() is useful for this.  As a
public function, it is renamed to qemu_iovec_slice().

(We will need to count the number of I/O vector elements of a slice
there, and then later process this slice.  Without qiov_slice(), we
would need to call qemu_iovec_subvec_niov(), and all further
IOV-processing functions may need to skip prefixing elements to
accomodate for a qiov_offset.  Because qemu_iovec_subvec_niov()
internally calls qiov_slice(), we can just have the block/io.c code call
qiov_slice() itself, thus get the number of elements, and also create an
iovec array with the superfluous prefixing elements stripped, so the
following processing functions no longer need to skip them.)

Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Vladimir Sementsov-Ogievskiy <vsementsov@yandex-team.ru>
Signed-off-by: Hanna Czenczek <hreitz@redhat.com>
Message-Id: <20230411173418.19549-2-hreitz@redhat.com>

include/qemu/iov.h

@@ -229,6 +229,9 @@ int qemu_iovec_init_extended(
         void *tail_buf, size_t tail_len);
 void qemu_iovec_init_slice(QEMUIOVector *qiov, QEMUIOVector *source,
                            size_t offset, size_t len);
+struct iovec *qemu_iovec_slice(QEMUIOVector *qiov,
+                               size_t offset, size_t len,
+                               size_t *head, size_t *tail, int *niov);
 int qemu_iovec_subvec_niov(QEMUIOVector *qiov, size_t offset, size_t len);
 void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len);
 void qemu_iovec_concat(QEMUIOVector *dst,

util/iov.c

@@ -378,15 +378,15 @@ static struct iovec *iov_skip_offset(struct iovec *iov, size_t offset,
 }
 
 /*
- * qiov_slice
+ * qemu_iovec_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 *qemu_iovec_slice(QEMUIOVector *qiov,
+                               size_t offset, size_t len,
+                               size_t *head, size_t *tail, int *niov)
 {
     struct iovec *iov, *end_iov;
 
@@ -411,7 +411,7 @@ int qemu_iovec_subvec_niov(QEMUIOVector *qiov, size_t offset, size_t len)
     size_t head, tail;
     int niov;
 
-    qiov_slice(qiov, offset, len, &head, &tail, &niov);
+    qemu_iovec_slice(qiov, offset, len, &head, &tail, &niov);
 
     return niov;
 }
@@ -439,8 +439,8 @@ int qemu_iovec_init_extended(
     }
 
     if (mid_len) {
-        mid_iov = qiov_slice(mid_qiov, mid_offset, mid_len,
-                             &mid_head, &mid_tail, &mid_niov);
+        mid_iov = qemu_iovec_slice(mid_qiov, mid_offset, mid_len,
+                                   &mid_head, &mid_tail, &mid_niov);
     }
 
     total_niov = !!head_len + mid_niov + !!tail_len;