[Qemu-devel] [PATCH 1/2] block: enhance QEMUIOVector structure

[Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>]

Add a possibility of embedded iovec, for cases when we need only one
local iov.

Signed-off-by: Vladimir Sementsov-Ogievskiy <vsementsov@virtuozzo.com>
---
 include/qemu/iov.h | 41 ++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 40 insertions(+), 1 deletion(-)

diff --git a/include/qemu/iov.h b/include/qemu/iov.h
index 5f433c7768..f739cff97d 100644
--- a/include/qemu/iov.h
+++ b/include/qemu/iov.h
@@ -134,9 +134,48 @@ typedef struct QEMUIOVector {
     struct iovec *iov;
     int niov;
     int nalloc;
-    size_t size;
+
+    /*
+     * For external @iov (qemu_iovec_init_external()) or allocated @iov
+     * (qemu_iovec_init()) @size is the cumulative size of iovecs and
+     * @local_iov is invalid and unused.
+     *
+     * For embedded @iov (QEMU_IOVEC_INIT_BUF() or qemu_iovec_init_buf()),
+     * @iov is equal to &@local_iov, and @size is valid, as it has same
+     * offset and type as @local_iov.iov_len, which is guaranteed by
+     * static assertions below.
+     */
+    union {
+        struct {
+            void *__unused_iov_base;
+            size_t size;
+        };
+        struct iovec local_iov;
+    };
 } QEMUIOVector;
 
+G_STATIC_ASSERT(offsetof(QEMUIOVector, size) ==
+                offsetof(QEMUIOVector, local_iov.iov_len));
+G_STATIC_ASSERT(sizeof(((QEMUIOVector *)NULL)->size) ==
+                sizeof(((QEMUIOVector *)NULL)->local_iov.iov_len));
+
+#define QEMU_IOVEC_INIT_BUF(self, buf, len) \
+{                                           \
+    .iov = &(self).local_iov,               \
+    .niov = 1,                              \
+    .nalloc = -1,                           \
+    .local_iov = {                          \
+        .iov_base = (void *)(buf),          \
+        .iov_len = len                      \
+    }                                       \
+}
+
+static inline void qemu_iovec_init_buf(QEMUIOVector *qiov,
+                                       void *buf, size_t len)
+{
+    *qiov = (QEMUIOVector) QEMU_IOVEC_INIT_BUF(*qiov, buf, len);
+}
+
 void qemu_iovec_init(QEMUIOVector *qiov, int alloc_hint);
 void qemu_iovec_init_external(QEMUIOVector *qiov, struct iovec *iov, int niov);
 void qemu_iovec_add(QEMUIOVector *qiov, void *base, size_t len);
-- 
2.18.0