Move copy out of qemu_peek_buffer

qemu_peek_buffer currently copies the data it reads into a buffer,
however a future patch wants access to the buffer without the copy,
hence rework to remove the copy to the layer above.
Signed-off-by: NDr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: NAmit Shah <amit.shah@redhat.com>
Reviewed-by: NJuan Quintela <quintela@redhat.com>
Signed-off-by: NJuan Quintela <quintela@redhat.com>

include/migration/qemu-file.h

@@ -157,7 +157,7 @@ static inline void qemu_put_ubyte(QEMUFile *f, unsigned int v)
 void qemu_put_be16(QEMUFile *f, unsigned int v);
 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_peek_buffer(QEMUFile *f, uint8_t **buf, int size, size_t offset);
 int qemu_get_buffer(QEMUFile *f, uint8_t *buf, int size);
 ssize_t qemu_put_compression_data(QEMUFile *f, const uint8_t *p, size_t size,
                                   int level);

migration/qemu-file.c

@@ -349,14 +349,14 @@ void qemu_file_skip(QEMUFile *f, int size)
 }
 
 /*
- * Read 'size' bytes from file (at 'offset') into buf without moving the
- * pointer.
+ * Read 'size' bytes from file (at 'offset') without moving the
+ * pointer and set 'buf' to point to that data.
  *
  * 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_peek_buffer(QEMUFile *f, uint8_t *buf, int size, size_t offset)
+int qemu_peek_buffer(QEMUFile *f, uint8_t **buf, int size, size_t offset)
 {
     int pending;
     int index;
@@ -392,7 +392,7 @@ int qemu_peek_buffer(QEMUFile *f, uint8_t *buf, int size, size_t offset)
         size = pending;
     }
 
-    memcpy(buf, f->buf + index, size);
+    *buf = f->buf + index;
     return size;
 }
 
@@ -411,11 +411,13 @@ int qemu_get_buffer(QEMUFile *f, uint8_t *buf, int size)
 
     while (pending > 0) {
         int res;
+        uint8_t *src;
 
-        res = qemu_peek_buffer(f, buf, MIN(pending, IO_BUF_SIZE), 0);
+        res = qemu_peek_buffer(f, &src, MIN(pending, IO_BUF_SIZE), 0);
         if (res == 0) {
             return done;
         }
+        memcpy(buf, src, res);
         qemu_file_skip(f, res);
         buf += res;
         pending -= res;

migration/vmstate.c

@@ -358,7 +358,7 @@ static int vmstate_subsection_load(QEMUFile *f, const VMStateDescription *vmsd,
     trace_vmstate_subsection_load(vmsd->name);
 
     while (qemu_peek_byte(f, 0) == QEMU_VM_SUBSECTION) {
-        char idstr[256];
+        char idstr[256], *idstr_ret;
         int ret;
         uint8_t version_id, len, size;
         const VMStateDescription *sub_vmsd;
@@ -369,11 +369,12 @@ static int vmstate_subsection_load(QEMUFile *f, const VMStateDescription *vmsd,
             trace_vmstate_subsection_load_bad(vmsd->name, "(short)");
             return 0;
         }
-        size = qemu_peek_buffer(f, (uint8_t *)idstr, len, 2);
+        size = qemu_peek_buffer(f, (uint8_t **)&idstr_ret, len, 2);
         if (size != len) {
             trace_vmstate_subsection_load_bad(vmsd->name, "(peek fail)");
             return 0;
         }
+        memcpy(idstr, idstr_ret, size);
         idstr[size] = 0;
 
         if (strncmp(vmsd->name, idstr, strlen(vmsd->name)) != 0) {