[Qemu-devel] [PATCH RFC] qemu-file: output data directly if possible

["Michael S. Tsirkin" <mst@redhat.com>]

qemu file currently always buffers up data before writing it out.
At least for memory this is probably not a good idea:
writing out to file would be cheaper. Let's do
that if we can, which should be the common case. If we can't, buffer.

Signed-off-by: Michael S. Tsirkin <mst@redhat.com>

---

Completely untested, this is just thinking aloud.
Shouldn't the below save us a data copy in the
common case, helping speed up migration?

Please comment.

diff --git a/qemu-file.c b/qemu-file.c
index 761f2a9..6d30151 100644
--- a/qemu-file.c
+++ b/qemu-file.c
@@ -142,6 +142,16 @@ void qemu_put_buffer(QEMUFile *f, const uint8_t *buf, int size)
         abort();
     }
 
+    if (!f->has_error && size > 0 && !f->buf_index) {
+        int len = f->put_buffer(f->opaque, buf, 0, size);
+        if (len >= 0) {
+            size -= len;
+            buf += len;
+        } else {
+            f->has_error = 1;
+        }
+    }
+
     while (!f->has_error && size > 0) {
         l = IO_BUF_SIZE - f->buf_index;
         if (l > size)