diff --git a/hw/block/block.c b/hw/block/block.c
index cf0eb826f1..bf56c7612b 100644
--- a/hw/block/block.c
+++ b/hw/block/block.c
@@ -13,7 +13,53 @@
 #include "hw/block/block.h"
 #include "qapi/error.h"
 #include "qapi/qapi-types-block.h"
-#include "qemu/error-report.h"
+
+/*
+ * Read the entire contents of @blk into @buf.
+ * @blk's contents must be @size bytes, and @size must be at most
+ * BDRV_REQUEST_MAX_BYTES.
+ * On success, return true.
+ * On failure, store an error through @errp and return false.
+ * Note that the error messages do not identify the block backend.
+ * TODO Since callers don't either, this can result in confusing
+ * errors.
+ * This function not intended for actual block devices, which read on
+ * demand.  It's for things like memory devices that (ab)use a block
+ * backend to provide persistence.
+ */
+bool blk_check_size_and_read_all(BlockBackend *blk, void *buf, hwaddr size,
+                                 Error **errp)
+{
+    int64_t blk_len;
+    int ret;
+
+    blk_len = blk_getlength(blk);
+    if (blk_len < 0) {
+        error_setg_errno(errp, -blk_len,
+                         "can't get size of block backend");
+        return false;
+    }
+    if (blk_len != size) {
+        error_setg(errp, "device requires %" HWADDR_PRIu " bytes, "
+                   "block backend provides %" PRIu64 " bytes",
+                   size, blk_len);
+        return false;
+    }
+
+    /*
+     * We could loop for @size > BDRV_REQUEST_MAX_BYTES, but if we
+     * ever get to the point we want to read *gigabytes* here, we
+     * should probably rework the device to be more like an actual
+     * block device and read only on demand.
+     */
+    assert(size <= BDRV_REQUEST_MAX_BYTES);
+    ret = blk_pread(blk, 0, buf, size);
+    if (ret < 0) {
+        error_setg_errno(errp, -ret, "can't read block backend");
+        return false;
+    }
+    return true;
+}
 
 void blkconf_blocksizes(BlockConf *conf)
 {
diff --git a/hw/block/pflash_cfi01.c b/hw/block/pflash_cfi01.c
index 125f70b8e4..9937739a82 100644
--- a/hw/block/pflash_cfi01.c
+++ b/hw/block/pflash_cfi01.c
@@ -38,6 +38,7 @@
 
 #include "qemu/osdep.h"
 #include "hw/hw.h"
+#include "hw/block/block.h"
 #include "hw/block/flash.h"
 #include "sysemu/block-backend.h"
 #include "qapi/error.h"
@@ -763,12 +764,9 @@ static void pflash_cfi01_realize(DeviceState *dev, Error **errp)
     }
 
     if (pfl->blk) {
-        /* read the initial flash content */
-        ret = blk_pread(pfl->blk, 0, pfl->storage, total_len);
-
-        if (ret < 0) {
+        if (!blk_check_size_and_read_all(pfl->blk, pfl->storage, total_len,
+                                         errp)) {
             vmstate_unregister_ram(&pfl->mem, DEVICE(pfl));
-            error_setg(errp, "failed to read the initial flash content");
             return;
         }
     }
diff --git a/hw/block/pflash_cfi02.c b/hw/block/pflash_cfi02.c
index c9db430611..9b934305fa 100644
--- a/hw/block/pflash_cfi02.c
+++ b/hw/block/pflash_cfi02.c
@@ -37,6 +37,7 @@
 
 #include "qemu/osdep.h"
 #include "hw/hw.h"
+#include "hw/block/block.h"
 #include "hw/block/flash.h"
 #include "qapi/error.h"
 #include "qemu/timer.h"
@@ -581,11 +582,9 @@ static void pflash_cfi02_realize(DeviceState *dev, Error **errp)
     }
 
     if (pfl->blk) {
-        /* read the initial flash content */
-        ret = blk_pread(pfl->blk, 0, pfl->storage, chip_len);
-        if (ret < 0) {
+        if (!blk_check_size_and_read_all(pfl->blk, pfl->storage, chip_len,
+                                         errp)) {
             vmstate_unregister_ram(&pfl->orig_mem, DEVICE(pfl));
-            error_setg(errp, "failed to read the initial flash content");
             return;
         }
     }
diff --git a/include/hw/block/block.h b/include/hw/block/block.h
index e9f9e2223f..d06f25aa0f 100644
--- a/include/hw/block/block.h
+++ b/include/hw/block/block.h
@@ -11,7 +11,7 @@
 #ifndef HW_BLOCK_H
 #define HW_BLOCK_H
 
-#include "qemu-common.h"
+#include "exec/hwaddr.h"
 #include "qapi/qapi-types-block-core.h"
 
 /* Configuration */
@@ -70,6 +70,11 @@ static inline unsigned int get_physical_block_exp(BlockConf *conf)
     DEFINE_PROP_BLOCKDEV_ON_ERROR("werror", _state, _conf.werror,       \
                                   BLOCKDEV_ON_ERROR_AUTO)
 
+/* Backend access helpers */
+
+bool blk_check_size_and_read_all(BlockBackend *blk, void *buf, hwaddr size,
+                                 Error **errp);
+
 /* Configuration helpers */
 
 bool blkconf_geometry(BlockConf *conf, int *trans,