block: add flag to indicate that no I/O will be performed

When opening an image it is useful to know whether the caller
intends to perform I/O on the image or not. In the case of
encrypted images this will allow the block driver to avoid
having to prompt for decryption keys when we merely want to
query header metadata about the image. eg qemu-img info

This flag is enforced at the top level only, since even if
we don't want todo I/O on the 'qcow2' file payload, the
underlying 'file' driver will still need todo I/O to read
the qcow2 header, for example.

Reviewed-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

4 files changed, 28 insertions, 24 deletions

diff --git a/block.c b/block.c
index 4158a3a36f..b4bec4b1e6 100644
--- a/block.c
+++ b/block.c
@@ -702,7 +702,8 @@ static void bdrv_inherited_options(int *child_flags, QDict *child_options,
     flags |= BDRV_O_UNMAP;
 
     /* Clear flags that only apply to the top layer */
-    flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ);
+    flags &= ~(BDRV_O_SNAPSHOT | BDRV_O_NO_BACKING | BDRV_O_COPY_ON_READ |
+               BDRV_O_NO_IO);
 
     *child_flags = flags;
 }
@@ -722,7 +723,7 @@ static void bdrv_inherited_fmt_options(int *child_flags, QDict *child_options,
     child_file.inherit_options(child_flags, child_options,
                                parent_flags, parent_options);
 
-    *child_flags &= ~BDRV_O_PROTOCOL;
+    *child_flags &= ~(BDRV_O_PROTOCOL | BDRV_O_NO_IO);
 }
 
 const BdrvChildRole child_format = {
diff --git a/block/io.c b/block/io.c
index c2611e53c8..4520cab852 100644
--- a/block/io.c
+++ b/block/io.c
@@ -844,6 +844,7 @@ static int coroutine_fn bdrv_aligned_preadv(BlockDriverState *bs,
     assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0);
     assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
     assert(!qiov || bytes == qiov->size);
+    assert((bs->open_flags & BDRV_O_NO_IO) == 0);
 
     /* Handle Copy on Read and associated serialisation */
     if (flags & BDRV_REQ_COPY_ON_READ) {
@@ -1130,6 +1131,7 @@ static int coroutine_fn bdrv_aligned_pwritev(BlockDriverState *bs,
     assert((offset & (BDRV_SECTOR_SIZE - 1)) == 0);
     assert((bytes & (BDRV_SECTOR_SIZE - 1)) == 0);
     assert(!qiov || bytes == qiov->size);
+    assert((bs->open_flags & BDRV_O_NO_IO) == 0);
 
     waited = wait_serialising_requests(req);
     assert(!waited || !req->serialising);
diff --git a/include/block/block.h b/include/block/block.h
index a48ad49397..c236e1d813 100644
--- a/include/block/block.h
+++ b/include/block/block.h
@@ -93,6 +93,7 @@ typedef struct HDGeometry {
 #define BDRV_O_PROTOCOL    0x8000  /* if no block driver is explicitly given:
                                       select an appropriate protocol driver,
                                       ignoring the format layer */
+#define BDRV_O_NO_IO       0x10000 /* don't initialize for I/O */
 
 #define BDRV_O_CACHE_MASK  (BDRV_O_NOCACHE | BDRV_O_CACHE_WB | BDRV_O_NO_FLUSH)
 
diff --git a/qemu-img.c b/qemu-img.c
index bd93d0a774..9e3ac9c1a8 100644
--- a/qemu-img.c
+++ b/qemu-img.c
@@ -225,13 +225,13 @@ static int print_block_option_help(const char *filename, const char *fmt)
 
 
 static int img_open_password(BlockBackend *blk, const char *filename,
-                             bool require_io, bool quiet)
+                             int flags, bool quiet)
 {
     BlockDriverState *bs;
     char password[256];
 
     bs = blk_bs(blk);
-    if (bdrv_is_encrypted(bs) && require_io) {
+    if (bdrv_is_encrypted(bs) && !(flags & BDRV_O_NO_IO)) {
         qprintf(quiet, "Disk image '%s' is encrypted.\n", filename);
         if (qemu_read_password(password, sizeof(password)) < 0) {
             error_report("No password given");
@@ -248,7 +248,7 @@ static int img_open_password(BlockBackend *blk, const char *filename,
 
 static BlockBackend *img_open_opts(const char *optstr,
                                    QemuOpts *opts, int flags,
-                                   bool require_io, bool quiet)
+                                   bool quiet)
 {
     QDict *options;
     Error *local_err = NULL;
@@ -260,7 +260,7 @@ static BlockBackend *img_open_opts(const char *optstr,
         return NULL;
     }
 
-    if (img_open_password(blk, optstr, require_io, quiet) < 0) {
+    if (img_open_password(blk, optstr, flags, quiet) < 0) {
         blk_unref(blk);
         return NULL;
     }
@@ -269,7 +269,7 @@ static BlockBackend *img_open_opts(const char *optstr,
 
 static BlockBackend *img_open_file(const char *filename,
                                    const char *fmt, int flags,
-                                   bool require_io, bool quiet)
+                                   bool quiet)
 {
     BlockBackend *blk;
     Error *local_err = NULL;
@@ -286,7 +286,7 @@ static BlockBackend *img_open_file(const char *filename,
         return NULL;
     }
 
-    if (img_open_password(blk, filename, require_io, quiet) < 0) {
+    if (img_open_password(blk, filename, flags, quiet) < 0) {
         blk_unref(blk);
         return NULL;
     }
@@ -297,7 +297,7 @@ static BlockBackend *img_open_file(const char *filename,
 static BlockBackend *img_open(bool image_opts,
                               const char *filename,
                               const char *fmt, int flags,
-                              bool require_io, bool quiet)
+                              bool quiet)
 {
     BlockBackend *blk;
     if (image_opts) {
@@ -311,9 +311,9 @@ static BlockBackend *img_open(bool image_opts,
         if (!opts) {
             return NULL;
         }
-        blk = img_open_opts(filename, opts, flags, true, quiet);
+        blk = img_open_opts(filename, opts, flags, quiet);
     } else {
-        blk = img_open_file(filename, fmt, flags, true, quiet);
+        blk = img_open_file(filename, fmt, flags, quiet);
     }
     return blk;
 }
@@ -685,7 +685,7 @@ static int img_check(int argc, char **argv)
         return 1;
     }
 
-    blk = img_open(image_opts, filename, fmt, flags, true, quiet);
+    blk = img_open(image_opts, filename, fmt, flags, quiet);
     if (!blk) {
         return 1;
     }
@@ -877,7 +877,7 @@ static int img_commit(int argc, char **argv)
         return 1;
     }
 
-    blk = img_open(image_opts, filename, fmt, flags, true, quiet);
+    blk = img_open(image_opts, filename, fmt, flags, quiet);
     if (!blk) {
         return 1;
     }
@@ -1211,13 +1211,13 @@ static int img_compare(int argc, char **argv)
         goto out3;
     }
 
-    blk1 = img_open(image_opts, filename1, fmt1, flags, true, quiet);
+    blk1 = img_open(image_opts, filename1, fmt1, flags, quiet);
     if (!blk1) {
         ret = 2;
         goto out3;
     }
 
-    blk2 = img_open(image_opts, filename2, fmt2, flags, true, quiet);
+    blk2 = img_open(image_opts, filename2, fmt2, flags, quiet);
     if (!blk2) {
         ret = 2;
         goto out2;
@@ -1899,7 +1899,7 @@ static int img_convert(int argc, char **argv)
     total_sectors = 0;
     for (bs_i = 0; bs_i < bs_n; bs_i++) {
         blk[bs_i] = img_open(image_opts, argv[optind + bs_i],
-                             fmt, src_flags, true, quiet);
+                             fmt, src_flags, quiet);
         if (!blk[bs_i]) {
             ret = -1;
             goto out;
@@ -2044,7 +2044,7 @@ static int img_convert(int argc, char **argv)
      * the bdrv_create() call which takes different params.
      * Not critical right now, so fix can wait...
      */
-    out_blk = img_open_file(out_filename, out_fmt, flags, true, quiet);
+    out_blk = img_open_file(out_filename, out_fmt, flags, quiet);
     if (!out_blk) {
         ret = -1;
         goto out;
@@ -2236,8 +2236,8 @@ static ImageInfoList *collect_image_info_list(bool image_opts,
         g_hash_table_insert(filenames, (gpointer)filename, NULL);
 
         blk = img_open(image_opts, filename, fmt,
-                       BDRV_O_FLAGS | BDRV_O_NO_BACKING,
-                       false, false);
+                       BDRV_O_FLAGS | BDRV_O_NO_BACKING | BDRV_O_NO_IO,
+                       false);
         if (!blk) {
             goto err;
         }
@@ -2567,7 +2567,7 @@ static int img_map(int argc, char **argv)
         return 1;
     }
 
-    blk = img_open(image_opts, filename, fmt, BDRV_O_FLAGS, true, false);
+    blk = img_open(image_opts, filename, fmt, BDRV_O_FLAGS, false);
     if (!blk) {
         return 1;
     }
@@ -2712,7 +2712,7 @@ static int img_snapshot(int argc, char **argv)
     }
 
     /* Open the image */
-    blk = img_open(image_opts, filename, NULL, bdrv_oflags, true, quiet);
+    blk = img_open(image_opts, filename, NULL, bdrv_oflags, quiet);
     if (!blk) {
         return 1;
     }
@@ -2883,7 +2883,7 @@ static int img_rebase(int argc, char **argv)
      * Ignore the old backing file for unsafe rebase in case we want to correct
      * the reference to a renamed or moved backing file.
      */
-    blk = img_open(image_opts, filename, fmt, flags, true, quiet);
+    blk = img_open(image_opts, filename, fmt, flags, quiet);
     if (!blk) {
         ret = -1;
         goto out;
@@ -3221,7 +3221,7 @@ static int img_resize(int argc, char **argv)
     qemu_opts_del(param);
 
     blk = img_open(image_opts, filename, fmt,
-                   BDRV_O_FLAGS | BDRV_O_RDWR, true, quiet);
+                   BDRV_O_FLAGS | BDRV_O_RDWR, quiet);
     if (!blk) {
         ret = -1;
         goto out;
@@ -3380,7 +3380,7 @@ static int img_amend(int argc, char **argv)
         goto out;
     }
 
-    blk = img_open(image_opts, filename, fmt, flags, true, quiet);
+    blk = img_open(image_opts, filename, fmt, flags, quiet);
     if (!blk) {
         ret = -1;
         goto out;