From: Xiang Zheng <zhengxia...@huawei.com>
Currently we fill the VIRT_FLASH memory space with two 64MB NOR images
when using persistent UEFI variables on virt board. Actually we only use
a very small(non-zero) part of the memory while the rest significant
large(zero) part of memory is wasted.
So this patch checks the block status and only writes the non-zero part
into memory. This requires pflash devices to use sparse files for
backends.
Signed-off-by: Xiang Zheng <zhengxia...@huawei.com>
[ kraxel: rebased to latest master ]
Signed-off-by: Gerd Hoffmann <kra...@redhat.com>
---
hw/block/block.c | 36 +++++++++++++++++++++++++++++++++++-
1 file changed, 35 insertions(+), 1 deletion(-)
diff --git a/hw/block/block.c b/hw/block/block.c
index f9c4fe67673b..142ebe4267e4 100644
--- a/hw/block/block.c
+++ b/hw/block/block.c
@@ -14,6 +14,40 @@
#include "qapi/error.h"
#include "qapi/qapi-types-block.h"
+/*
+ * Read the non-zeroes parts of @blk into @buf
+ * Reading all of the @blk is expensive if the zeroes parts of @blk
+ * is large enough. Therefore check the block status and only write
+ * the non-zeroes block into @buf.
+ *
+ * Return 0 on success, non-zero on error.
+ */
+static int blk_pread_nonzeroes(BlockBackend *blk, hwaddr size, void *buf)
+{
+ int ret;
+ int64_t bytes, offset = 0;
+ BlockDriverState *bs = blk_bs(blk);
+
+ for (;;) {
+ bytes = MIN(size - offset, BDRV_REQUEST_MAX_SECTORS);
+ if (bytes <= 0) {
+ return 0;
+ }
+ ret = bdrv_block_status(bs, offset, bytes, &bytes, NULL, NULL);
+ if (ret < 0) {
+ return ret;
+ }
+ if (!(ret & BDRV_BLOCK_ZERO)) {
+ ret = bdrv_pread(bs->file, offset, bytes,
+ (uint8_t *) buf + offset, 0);
+ if (ret < 0) {
+ return ret;
+ }
+ }
+ offset += bytes;
+ }
+}
+
/*
* Read the entire contents of @blk into @buf.
* @blk's contents must be @size bytes, and @size must be at most
@@ -53,7 +87,7 @@ bool blk_check_size_and_read_all(BlockBackend *blk, void
*buf, hwaddr size,
* block device and read only on demand.
*/
assert(size <= BDRV_REQUEST_MAX_BYTES);
- ret = blk_pread(blk, 0, size, buf, 0);
+ ret = blk_pread_nonzeroes(blk, size, buf);
if (ret < 0) {
error_setg_errno(errp, -ret, "can't read block backend");
return false;
--
2.38.1
