block: pass size to bdrv_unregister_buf()

The only implementor of bdrv_register_buf() is block/nvme.c, where the
size is not needed when unregistering a buffer. This is because
util/vfio-helpers.c can look up mappings by address.

Future block drivers that implement bdrv_register_buf() may not be able
to do their job given only the buffer address. Add a size argument to
bdrv_unregister_buf().

Also document the assumptions about
bdrv_register_buf()/bdrv_unregister_buf() calls. The same <host, size>
values that were given to bdrv_register_buf() must be given to
bdrv_unregister_buf().

gcc 11.2.1 emits a spurious warning that img_bench()'s buf_size local
variable might be uninitialized, so it's necessary to silence the
compiler.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>
Message-id: 20221013185908.1297568-5-stefanha@redhat.com
Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>

block/block-backend.c

@@ -2551,10 +2551,10 @@ void blk_register_buf(BlockBackend *blk, void *host, size_t size)
     bdrv_register_buf(blk_bs(blk), host, size);
 }
 
-void blk_unregister_buf(BlockBackend *blk, void *host)
+void blk_unregister_buf(BlockBackend *blk, void *host, size_t size)
 {
     GLOBAL_STATE_CODE();
-    bdrv_unregister_buf(blk_bs(blk), host);
+    bdrv_unregister_buf(blk_bs(blk), host, size);
 }
 
 int coroutine_fn blk_co_copy_range(BlockBackend *blk_in, int64_t off_in,

block/io.c

@@ -3275,16 +3275,16 @@ void bdrv_register_buf(BlockDriverState *bs, void *host, size_t size)
     }
 }
 
-void bdrv_unregister_buf(BlockDriverState *bs, void *host)
+void bdrv_unregister_buf(BlockDriverState *bs, void *host, size_t size)
 {
     BdrvChild *child;
 
     GLOBAL_STATE_CODE();
     if (bs->drv && bs->drv->bdrv_unregister_buf) {
-        bs->drv->bdrv_unregister_buf(bs, host);
+        bs->drv->bdrv_unregister_buf(bs, host, size);
     }
     QLIST_FOREACH(child, &bs->children, next) {
-        bdrv_unregister_buf(child->bs, host);
+        bdrv_unregister_buf(child->bs, host, size);
     }
 }
 

block/nvme.c

@@ -1602,7 +1602,7 @@ static void nvme_register_buf(BlockDriverState *bs, void *host, size_t size)
     }
 }
 
-static void nvme_unregister_buf(BlockDriverState *bs, void *host)
+static void nvme_unregister_buf(BlockDriverState *bs, void *host, size_t size)
 {
     BDRVNVMeState *s = bs->opaque;
 

include/block/block-global-state.h

@@ -243,9 +243,12 @@ void bdrv_del_child(BlockDriverState *parent, BdrvChild *child, Error **errp);
  * Register/unregister a buffer for I/O. For example, VFIO drivers are
  * interested to know the memory areas that would later be used for I/O, so
  * that they can prepare IOMMU mapping etc., to get better performance.
+ *
+ * Buffers must not overlap and they must be unregistered with the same <host,
+ * size> values that they were registered with.
  */
 void bdrv_register_buf(BlockDriverState *bs, void *host, size_t size);
-void bdrv_unregister_buf(BlockDriverState *bs, void *host);
+void bdrv_unregister_buf(BlockDriverState *bs, void *host, size_t size);
 
 void bdrv_cancel_in_flight(BlockDriverState *bs);
 

include/block/block_int-common.h

@@ -435,7 +435,7 @@ struct BlockDriver {
      * DMA mapping for hot buffers.
      */
     void (*bdrv_register_buf)(BlockDriverState *bs, void *host, size_t size);
-    void (*bdrv_unregister_buf)(BlockDriverState *bs, void *host);
+    void (*bdrv_unregister_buf)(BlockDriverState *bs, void *host, size_t size);
 
     /*
      * This field is modified only under the BQL, and is part of

include/sysemu/block-backend-global-state.h

@@ -107,7 +107,7 @@ void blk_io_limits_update_group(BlockBackend *blk, const char *group);
 void blk_set_force_allow_inactivate(BlockBackend *blk);
 
 void blk_register_buf(BlockBackend *blk, void *host, size_t size);
-void blk_unregister_buf(BlockBackend *blk, void *host);
+void blk_unregister_buf(BlockBackend *blk, void *host, size_t size);
 
 const BdrvChild *blk_root(BlockBackend *blk);
 

qemu-img.c

@@ -4371,7 +4371,7 @@ static int img_bench(int argc, char **argv)
     struct timeval t1, t2;
     int i;
     bool force_share = false;
-    size_t buf_size;
+    size_t buf_size = 0;
 
     for (;;) {
         static const struct option long_options[] = {
@@ -4593,7 +4593,7 @@ static int img_bench(int argc, char **argv)
 
 out:
     if (data.buf) {
-        blk_unregister_buf(blk, data.buf);
+        blk_unregister_buf(blk, data.buf, buf_size);
     }
     qemu_vfree(data.buf);
     blk_unref(blk);