block: move has_variable_length to BlockLimits

At the protocol level, has_variable_length only needs to be true in the
very special case of host CD-ROM drives, so that they do not need an
explicit monitor command to read the new size when a disc is loaded
in the tray.

However, at the format level has_variable_length has to be true for all
raw blockdevs and for all filters, even though in practice the length
depends on the underlying file and thus will not change except in the
case of host CD-ROM drives.

As a first step towards computing an accurate value of has_variable_length,
add the value into the BlockLimits structure and initialize the field
from the BlockDriver.

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Message-Id: <20230407153303.391121-2-pbonzini@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

diff --git a/block.c b/block.c
index e0c6c648b129e5d1579f96e68a023accd31d912f..6a805ff0ea8b703a0c2048a533d1f42caf343956 100644 (file)
--- a/block.c
+++ b/block.c
@@ -5849,7 +5849,7 @@ int64_t coroutine_fn bdrv_co_nb_sectors(BlockDriverState *bs)
     if (!drv)
         return -ENOMEDIUM;
 
-    if (drv->has_variable_length) {
+    if (bs->bl.has_variable_length) {
         int ret = bdrv_co_refresh_total_sectors(bs, bs->total_sectors);
         if (ret < 0) {
             return ret;

diff --git a/block/io.c b/block/io.c
index 8974d46941b55b2a2bfec569bd099f40a0acf174..932aeb5844e20f7ae1b8f56c55d0d4fd61650fb5 100644 (file)
--- a/block/io.c
+++ b/block/io.c
@@ -182,6 +182,8 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp)
                                 drv->bdrv_aio_preadv ||
                                 drv->bdrv_co_preadv_part) ? 1 : 512;
 
+    bs->bl.has_variable_length = drv->has_variable_length;
+
     /* Take some limits from the children as a default */
     have_limits = false;
     QLIST_FOREACH(c, &bs->children, next) {
@@ -190,6 +192,10 @@ void bdrv_refresh_limits(BlockDriverState *bs, Transaction *tran, Error **errp)
             bdrv_merge_limits(&bs->bl, &c->bs->bl);
             have_limits = true;
         }
+
+        if (c->role & BDRV_CHILD_FILTERED) {
+            bs->bl.has_variable_length |= c->bs->bl.has_variable_length;
+        }
     }
 
     if (!have_limits) {

diff --git a/include/block/block_int-common.h b/include/block/block_int-common.h
index d4190173284fa37b4053b814893bb9c2c8c06981..a6d271f25dee5407ebf63d83b835465a3506668b 100644 (file)
--- a/include/block/block_int-common.h
+++ b/include/block/block_int-common.h
@@ -855,6 +855,14 @@ typedef struct BlockLimits {
 
     /* maximum number of iovec elements */
     int max_iov;
+
+    /*
+     * true if the length of the underlying file can change, and QEMU
+     * is expected to adjust automatically.  Mostly for CD-ROM drives,
+     * whose length is zero when the tray is empty (they don't need
+     * an explicit monitor command to load the disk inside the guest).
+     */
+    bool has_variable_length;
 } BlockLimits;
 
 typedef struct BdrvOpBlocker BdrvOpBlocker;