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));
