[PATCH v7 2/6] libs/ctrl: Allow writev_exact() to change the iov array

[Frediano Ziglio <freddy77@xxxxxxxxx>]

Avoid having to allocate and copy the array if a partial write happens.

Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
---
Changes since v2:
- change prefix in subject.

Changes since v6:
- use a single loop to skip IOVs;
- add a comment to the function.
---
 tools/libs/ctrl/xc_private.c | 65 +++++++++---------------------------
 tools/libs/ctrl/xc_private.h |  3 +-
 2 files changed, 17 insertions(+), 51 deletions(-)

diff --git a/tools/libs/ctrl/xc_private.c b/tools/libs/ctrl/xc_private.c
index bb0f81d6f3..89ec68a183 100644
--- a/tools/libs/ctrl/xc_private.c
+++ b/tools/libs/ctrl/xc_private.c
@@ -635,7 +635,7 @@ int write_exact(int fd, const void *data, size_t size)
 /*
  * MiniOS's libc doesn't know about writev(). Implement it as multiple 
write()s.
  */
-int writev_exact(int fd, const struct iovec *iov, int iovcnt)
+int writev_exact(int fd, struct iovec *iov, int iovcnt)
 {
     int rc, i;
 
@@ -649,70 +649,35 @@ int writev_exact(int fd, const struct iovec *iov, int 
iovcnt)
     return 0;
 }
 #else
-int writev_exact(int fd, const struct iovec *iov, int iovcnt)
+int writev_exact(int fd, struct iovec *iov, int iovcnt)
 {
-    struct iovec *local_iov = NULL;
-    int rc = 0, iov_idx = 0, saved_errno = 0;
-    ssize_t len;
+    int rc = 0, iov_idx = 0;
+    ssize_t len = 0;
 
     while ( iov_idx < iovcnt )
     {
-        /*
-         * Skip over iov[] entries with 0 length.
-         *
-         * This is needed to cover the case where we took a partial write and
-         * all remaining vectors are of 0 length.  In such a case, the results
-         * from writev() are indistinguishable from EOF.
-         */
-        while ( iov[iov_idx].iov_len == 0 )
-            if ( ++iov_idx == iovcnt )
-                goto out;
+        /* Check iov[] to see whether we had a partial or complete write. */
+        if ( len >= iov[iov_idx].iov_len )
+        {
+            len -= iov[iov_idx++].iov_len;
+            continue;
+        }
+
+        /* Partial write of iov[iov_idx]. */
+        iov[iov_idx].iov_base += len;
+        iov[iov_idx].iov_len  -= len;
 
         len = writev(fd, &iov[iov_idx], min(iovcnt - iov_idx, IOV_MAX));
-        saved_errno = errno;
 
         if ( (len == -1) && (errno == EINTR) )
             continue;
         if ( len <= 0 )
         {
             rc = -1;
-            goto out;
-        }
-
-        /* Check iov[] to see whether we had a partial or complete write. */
-        while ( (len > 0) && (iov_idx < iovcnt) )
-        {
-            if ( len >= iov[iov_idx].iov_len )
-                len -= iov[iov_idx++].iov_len;
-            else
-            {
-                /* Partial write of iov[iov_idx]. Copy iov so we can adjust
-                 * element iov_idx and resubmit the rest. */
-                if ( !local_iov )
-                {
-                    local_iov = malloc(iovcnt * sizeof(*iov));
-                    if ( !local_iov )
-                    {
-                        saved_errno = ENOMEM;
-                        rc = -1;
-                        goto out;
-                    }
-
-                    iov = memcpy(local_iov, iov, iovcnt * sizeof(*iov));
-                }
-
-                local_iov[iov_idx].iov_base += len;
-                local_iov[iov_idx].iov_len  -= len;
-                break;
-            }
+            break;
         }
     }
 
-    saved_errno = 0;
-
- out:
-    free(local_iov);
-    errno = saved_errno;
     return rc;
 }
 #endif
diff --git a/tools/libs/ctrl/xc_private.h b/tools/libs/ctrl/xc_private.h
index b5892ae8dc..8a325c17b0 100644
--- a/tools/libs/ctrl/xc_private.h
+++ b/tools/libs/ctrl/xc_private.h
@@ -383,7 +383,8 @@ int xc_flush_mmu_updates(xc_interface *xch, struct xc_mmu 
*mmu);
 /* Return 0 on success; -1 on error setting errno. */
 int read_exact(int fd, void *data, size_t size); /* EOF => -1, errno=0 */
 int write_exact(int fd, const void *data, size_t size);
-int writev_exact(int fd, const struct iovec *iov, int iovcnt);
+/* May alter @iov to cope with partial writes. */
+int writev_exact(int fd, struct iovec *iov, int iovcnt);
 
 int xc_ffs8(uint8_t x);
 int xc_ffs16(uint16_t x);
-- 
2.43.0