pflash: Only read non-zero parts of backend image

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 <zhengxiang9@huawei.com>

[ kraxel: rebased to latest master ]

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Message-Id: <20221220084246.1984871-1-kraxel@redhat.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

hw/block/block.c

@@ -15,6 +15,40 @@
 #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
@@ -54,7 +88,7 @@ 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;