From: "Dr. David Alan Gilbert" <dgilb...@redhat.com>
qemu_get_buffer always copies the data it reads to a users buffer,
however in many cases the file buffer inside qemu_file could be given
back to the caller, avoiding the copy. This isn't always possible
depending on the size and alignment of the data.
Thus 'qemu_get_buffer_less_copy' either copies the data to a supplied
buffer or updates a pointer to the internal buffer if convenient.
Signed-off-by: Dr. David Alan Gilbert <dgilb...@redhat.com>
---
include/migration/qemu-file.h | 2 ++
migration/qemu-file.c | 45 +++++++++++++++++++++++++++++++++++++++++++
2 files changed, 47 insertions(+)
diff --git a/include/migration/qemu-file.h b/include/migration/qemu-file.h
index 3fe545e..4cac58f 100644
--- a/include/migration/qemu-file.h
+++ b/include/migration/qemu-file.h
@@ -159,6 +159,8 @@ void qemu_put_be32(QEMUFile *f, unsigned int v);
void qemu_put_be64(QEMUFile *f, uint64_t v);
int qemu_peek_buffer(QEMUFile *f, uint8_t **buf, int size, size_t offset);
int qemu_get_buffer(QEMUFile *f, uint8_t *buf, int size);
+int qemu_get_buffer_less_copy(QEMUFile *f, uint8_t **buf, int size);
+
/*
* Note that you can only peek continuous bytes from where the current pointer
* is; you aren't guaranteed to be able to peak to +n bytes unless you've
diff --git a/migration/qemu-file.c b/migration/qemu-file.c
index 8dc5767..ec3a598 100644
--- a/migration/qemu-file.c
+++ b/migration/qemu-file.c
@@ -426,6 +426,51 @@ int qemu_get_buffer(QEMUFile *f, uint8_t *buf, int size)
}
/*
+ * Read 'size' bytes of data from the file.
+ * 'size' can be larger than the internal buffer.
+ *
+ * The data:
+ * may be held on an internal buffer (in which case *buf is updated
+ * to point to it) that is valid until the next qemu_file operation.
+ * OR
+ * will be copied to the *buf that was passed in.
+ *
+ * The code tries to avoid the copy if possible.
+ *
+ * It will return size bytes unless there was an error, in which case it will
+ * return as many as it managed to read (assuming blocking fd's which
+ * all current QEMUFile are)
+ */
+int qemu_get_buffer_less_copy(QEMUFile *f, uint8_t **buf, int size)
+{
+ int pending = size;
+ int done = 0;
+ bool first = true;
+
+ while (pending > 0) {
+ int res;
+ uint8_t *src;
+
+ res = qemu_peek_buffer(f, &src, MIN(pending, IO_BUF_SIZE), 0);
+ if (res == 0) {
+ return done;
+ }
+ qemu_file_skip(f, res);
+ done += res;
+ pending -= res;
+ if (first && res == size) {
+ *buf = src;
+ return done;
+ } else {
+ first = false;
+ memcpy(buf, src, res);
+ buf += res;
+ }
+ }
+ return done;
+}
+
+/*
* Peeks a single byte from the buffer; this isn't guaranteed to work if
* offset leaves a gap after the previous read/peeked data.
*/
--
2.1.0
