[PATCH v5 02/16] libs/guest: Reduce number of I/O vectors in write_batch

[Frediano Ziglio <freddy77@gmail.com>]

From: Frediano Ziglio <frediano.ziglio@citrix.com>

Small optimization.
Reduce number of pieces passed to writev.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
--
Changes since v2:
- change prefix in subject.

Changes since v4:
- added Reviewed-by.
---
 tools/libs/guest/xg_sr_save.c | 34 +++++++++++++++-------------------
 1 file changed, 15 insertions(+), 19 deletions(-)

diff --git a/tools/libs/guest/xg_sr_save.c b/tools/libs/guest/xg_sr_save.c
index fdbceab52e..68ce1aeb98 100644
--- a/tools/libs/guest/xg_sr_save.c
+++ b/tools/libs/guest/xg_sr_save.c
@@ -97,9 +97,11 @@ static int write_batch(struct xc_sr_context *ctx)
     void *page, *orig_page;
     uint64_t *rec_pfns = NULL;
     struct iovec *iov = NULL; int iovcnt = 0;
-    struct xc_sr_rec_page_data_header hdr = { 0 };
-    struct xc_sr_record rec = {
-        .type = REC_TYPE_PAGE_DATA,
+    struct {
+        struct xc_sr_rhdr rec;
+        struct xc_sr_rec_page_data_header page_data;
+    } hdrs = {
+        { .type = REC_TYPE_PAGE_DATA },
     };
 
     assert(nr_pfns != 0);
@@ -115,7 +117,7 @@ static int write_batch(struct xc_sr_context *ctx)
     /* Pointers to locally allocated pages.  Need freeing. */
     local_pages = calloc(nr_pfns, sizeof(*local_pages));
     /* iovec[] for writev(). */
-    iov = malloc((nr_pfns + 4) * sizeof(*iov));
+    iov = malloc((nr_pfns + 2) * sizeof(*iov));
 
     if ( !mfns || !types || !errors || !guest_data || !local_pages || !iov )
     {
@@ -216,28 +218,22 @@ static int write_batch(struct xc_sr_context *ctx)
         goto err;
     }
 
-    hdr.count = nr_pfns;
+    hdrs.rec.length = sizeof(hdrs.page_data);
+    hdrs.rec.length += nr_pfns * sizeof(*rec_pfns);
+    hdrs.rec.length += nr_pages * PAGE_SIZE;
 
-    rec.length = sizeof(hdr);
-    rec.length += nr_pfns * sizeof(*rec_pfns);
-    rec.length += nr_pages * PAGE_SIZE;
+    hdrs.page_data.count = nr_pfns;
 
     for ( i = 0; i < nr_pfns; ++i )
         rec_pfns[i] = ((uint64_t)(types[i]) << 32) | ctx->save.batch_pfns[i];
 
-    iov[0].iov_base = &rec.type;
-    iov[0].iov_len = sizeof(rec.type);
+    iov[0].iov_base = &hdrs;
+    iov[0].iov_len = sizeof(hdrs);
 
-    iov[1].iov_base = &rec.length;
-    iov[1].iov_len = sizeof(rec.length);
+    iov[1].iov_base = rec_pfns;
+    iov[1].iov_len = nr_pfns * sizeof(*rec_pfns);
 
-    iov[2].iov_base = &hdr;
-    iov[2].iov_len = sizeof(hdr);
-
-    iov[3].iov_base = rec_pfns;
-    iov[3].iov_len = nr_pfns * sizeof(*rec_pfns);
-
-    iovcnt = 4;
+    iovcnt = 2;
 
     if ( nr_pages )
     {
-- 
2.43.0