libs/guest: Use a single write_exact() in write_headers()

Reduce number of syscalls by coalescing the image and the domain headers.

Signed-off-by: Frediano Ziglio <frediano.ziglio@citrix.com>
Reviewed-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Anthony PERARD <anthony.perard@vates.tech>

tools/libs/guest/xg_sr_save.c

@@ -10,17 +10,22 @@ static int write_headers(struct xc_sr_context *ctx, uint16_t guest_type)
 {
     xc_interface *xch = ctx->xch;
     int32_t xen_version = xc_version(xch, XENVER_version, NULL);
-    struct xc_sr_ihdr ihdr = {
-        .marker  = IHDR_MARKER,
-        .id      = htonl(IHDR_ID),
-        .version = htonl(3),
-        .options = htons(IHDR_OPT_LITTLE_ENDIAN),
-    };
-    struct xc_sr_dhdr dhdr = {
-        .type       = guest_type,
-        .page_shift = XC_PAGE_SHIFT,
-        .xen_major  = (xen_version >> 16) & 0xffff,
-        .xen_minor  = (xen_version)       & 0xffff,
+    struct {
+        struct xc_sr_ihdr ihdr;
+        struct xc_sr_dhdr dhdr;
+    } hdrs = {
+        .ihdr = {
+            .marker  = IHDR_MARKER,
+            .id      = htonl(IHDR_ID),
+            .version = htonl(3),
+            .options = htons(IHDR_OPT_LITTLE_ENDIAN),
+        },
+        .dhdr = {
+            .type       = guest_type,
+            .page_shift = XC_PAGE_SHIFT,
+            .xen_major  = (xen_version >> 16) & 0xffff,
+            .xen_minor  = (xen_version)       & 0xffff,
+        },
     };
 
     if ( xen_version < 0 )
@@ -29,15 +34,9 @@ static int write_headers(struct xc_sr_context *ctx, uint16_t guest_type)
         return -1;
     }
 
-    if ( write_exact(ctx->fd, &ihdr, sizeof(ihdr)) )
+    if ( write_exact(ctx->fd, &hdrs, sizeof(hdrs)) )
     {
-        PERROR("Unable to write Image Header to stream");
-        return -1;
-    }
-
-    if ( write_exact(ctx->fd, &dhdr, sizeof(dhdr)) )
-    {
-        PERROR("Unable to write Domain Header to stream");
+        PERROR("Unable to write Image/Domain headers to stream");
         return -1;
     }