block: bump coroutine pool size for drives

When a BlockDriverState is associated with a storage controller
DeviceState we expect guest I/O.  Use this opportunity to bump the
coroutine pool size by 64.

This patch ensures that the coroutine pool size scales with the number
of drives attached to the guest.  It should increase coroutine pool
usage (which makes qemu_coroutine_create() fast) without hogging too
much memory when fewer drives are attached.
Signed-off-by: NStefan Hajnoczi <stefanha@redhat.com>
Reviewed-by: NEric Blake <eblake@redhat.com>

block.c

@@ -57,6 +57,8 @@ struct BdrvDirtyBitmap {
 
 #define NOT_DONE 0x7fffffff /* used while emulated sync operation in progress */
 
+#define COROUTINE_POOL_RESERVATION 64 /* number of coroutines to reserve */
+
 static void bdrv_dev_change_media_cb(BlockDriverState *bs, bool load);
 static BlockDriverAIOCB *bdrv_aio_readv_em(BlockDriverState *bs,
         int64_t sector_num, QEMUIOVector *qiov, int nb_sectors,
@@ -2107,6 +2109,9 @@ int bdrv_attach_dev(BlockDriverState *bs, void *dev)
     }
     bs->dev = dev;
     bdrv_iostatus_reset(bs);
+
+    /* We're expecting I/O from the device so bump up coroutine pool size */
+    qemu_coroutine_adjust_pool_size(COROUTINE_POOL_RESERVATION);
     return 0;
 }
 
@@ -2126,6 +2131,7 @@ void bdrv_detach_dev(BlockDriverState *bs, void *dev)
     bs->dev_ops = NULL;
     bs->dev_opaque = NULL;
     bs->guest_block_size = 512;
+    qemu_coroutine_adjust_pool_size(-COROUTINE_POOL_RESERVATION);
 }
 
 /* TODO change to return DeviceState * when all users are qdevified */