[PATCH v6.1 02/16] libs/guest: Reduce number of iovecs in write_batch()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

From: Frediano Ziglio <freddy77@xxxxxxxxx>

Construct all of the headers together in one block, rather than a field at a
time.  Initialise as many of the fields as possible at declaration time.

Start filling in iov[] earlier, to allow for future simplifications.

No practical change.

Signed-off-by: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
CC: Anthony PERARD <anthony.perard@xxxxxxxxxx>
CC: Frediano Ziglio <frediano.ziglio@xxxxxxxxxx>
---
 tools/libs/guest/xg_sr_save.c | 45 +++++++++++++++++------------------
 1 file changed, 22 insertions(+), 23 deletions(-)

diff --git a/tools/libs/guest/xg_sr_save.c b/tools/libs/guest/xg_sr_save.c
index 69fe991a8113..7736f4a055e0 100644
--- a/tools/libs/guest/xg_sr_save.c
+++ b/tools/libs/guest/xg_sr_save.c
@@ -97,9 +97,18 @@ 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 = {
+        .rec = {
+            .type = REC_TYPE_PAGE_DATA,
+            .length = offsetof(struct xc_sr_rec_page_data_header,
+                               pfn[nr_pfns]), /* + the pages to send */
+        },
+        .page_data = {
+            .count = nr_pfns,
+        },
     };
 
     assert(nr_pfns != 0);
@@ -115,7 +124,7 @@ 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));
     /* page_data record PFNs list */
     rec_pfns = malloc(nr_pfns * sizeof(*rec_pfns));
 
@@ -126,6 +135,14 @@ static int write_batch(struct xc_sr_context *ctx)
         goto err;
     }
 
+    iov[0].iov_base = &hdrs;
+    iov[0].iov_len = sizeof(hdrs);
+
+    iov[1].iov_base = rec_pfns;
+    iov[1].iov_len = nr_pfns * sizeof(*rec_pfns);
+
+    iovcnt = 2;
+
     for ( i = 0; i < nr_pfns; ++i )
     {
         types[i] = mfns[i] = ctx->save.ops.pfn_to_gfn(ctx,
@@ -210,29 +227,11 @@ static int write_batch(struct xc_sr_context *ctx)
         }
     }
 
-    hdr.count = nr_pfns;
-
-    rec.length = sizeof(hdr);
-    rec.length += nr_pfns * sizeof(*rec_pfns);
-    rec.length += nr_pages * PAGE_SIZE;
+    hdrs.rec.length += nr_pages * PAGE_SIZE;
 
     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[1].iov_base = &rec.length;
-    iov[1].iov_len = sizeof(rec.length);
-
-    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;
-
     if ( nr_pages )
     {
         for ( i = 0; i < nr_pfns; ++i )
-- 
2.39.5