[PATCH v1.1 5/6] tools/libxs: Use writev()/sendmsg() instead of write()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

With the input data now conveniently arranged, use writev()/sendmsg() instead
of decomposing it into write() calls.

This causes all requests to be submitted with a single system call, rather
than at least two.  While in principle short writes can occur, the chances of
it happening are slim given that most xenbus comms are only a handful of
bytes.

Nevertheless, provide {writev,sendmsg}_exact() wrappers which take care of
resubmitting on EINTR or short write.

Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
CC: Juergen Gross <jgross@xxxxxxxx>
CC: Frediano Ziglio <frediano.ziglio@xxxxxxxxx>

v1.1:
 * Fix iov overread, spotted by Frediano.  Factor the common updating logic
   out into update_iov().
---
 tools/libs/store/xs.c | 94 +++++++++++++++++++++++++++++++++++++++++--
 1 file changed, 91 insertions(+), 3 deletions(-)

diff --git a/tools/libs/store/xs.c b/tools/libs/store/xs.c
index e820cccc2314..f80ac7558cbe 100644
--- a/tools/libs/store/xs.c
+++ b/tools/libs/store/xs.c
@@ -563,6 +563,95 @@ static void *read_reply(
        return body;
 }
 
+/*
+ * Update an iov/nr pair after an incomplete writev()/sendmsg().
+ *
+ * Awkwardly, nr has different widths and signs between writev() and
+ * sendmsg(), so we take it and return it by value, rather than by pointer.
+ */
+static size_t update_iov(struct iovec **p_iov, size_t nr, size_t res)
+{
+       struct iovec *iov = *p_iov;
+
+        /* Skip fully complete elements, including empty elements. */
+        while (nr && res >= iov->iov_len) {
+                res -= iov->iov_len;
+                nr--;
+                iov++;
+        }
+
+        /* Partial element, adjust base/len. */
+        if (res) {
+                iov->iov_len  -= res;
+                iov->iov_base += res;
+        }
+
+        *p_iov = iov;
+
+       return nr;
+}
+
+/*
+ * Wrapper around sendmsg() to resubmit on EINTR or short write.  Returns
+ * @true if all data was transmitted, or @false with errno for an error.
+ * Note: May alter @iov in place on resubmit.
+ */
+static bool sendmsg_exact(int fd, struct iovec *iov, unsigned int nr)
+{
+       struct msghdr hdr = {
+               .msg_iov = iov,
+               .msg_iovlen = nr,
+       };
+
+       /* Sanity check first element isn't empty */
+       assert(iov->iov_len == sizeof(struct xsd_sockmsg));
+
+       while (hdr.msg_iovlen) {
+               ssize_t res = sendmsg(fd, &hdr, 0);
+
+               if (res < 0 && errno == EINTR)
+                       continue;
+               if (res <= 0)
+                       return false;
+
+               hdr.msg_iovlen = update_iov(&hdr.msg_iov, hdr.msg_iovlen, res);
+       }
+
+       return true;
+}
+
+/*
+ * Wrapper around sendmsg() to resubmit on EINTR or short write.  Returns
+ * @true if all data was transmitted, or @false with errno for an error.
+ * Note: May alter @iov in place on resubmit.
+ */
+static bool writev_exact(int fd, struct iovec *iov, unsigned int nr)
+{
+       /* Sanity check first element isn't empty */
+       assert(iov->iov_len == sizeof(struct xsd_sockmsg));
+
+       while (nr) {
+               ssize_t res = writev(fd, iov, nr);
+
+               if (res < 0 && errno == EINTR)
+                       continue;
+               if (res <= 0)
+                       return false;
+
+               nr = update_iov(&iov, nr, res);
+       }
+
+       return true;
+}
+
+static bool write_request(struct xs_handle *h, struct iovec *iov, unsigned int 
nr)
+{
+       if (h->is_socket)
+               return sendmsg_exact(h->fd, iov, nr);
+       else
+               return writev_exact(h->fd, iov, nr);
+}
+
 /*
  * Send message to xenstore, get malloc'ed reply.  NULL and set errno on error.
  *
@@ -605,9 +694,8 @@ static void *xs_talkv(struct xs_handle *h,
 
        mutex_lock(&h->request_mutex);
 
-       for (i = 0; i < num_vecs; i++)
-               if (!xs_write_all(h->fd, iovec[i].iov_base, iovec[i].iov_len))
-                       goto fail;
+       if (!write_request(h, iovec, num_vecs))
+               goto fail;
 
        ret = read_reply(h, &reply_type, len);
        if (!ret)
-- 
2.39.2