Question about QMP and BQL

[Fabiano Rosas]

Is there a way to execute a long-standing QMP command outside of the
BQL?

The situation we're seeing is a slow query-block due to a slow system
call (fstat over NFS) causing the main thread to spend too long
holding the global mutex and locking up the vcpu thread when it goes
out of the guest for MMIO.

The call chain for QMP is:

qmp_query_block
bdrv_query_info
bdrv_block_device_info
bdrv_query_image_info
bdrv_do_query_node_info
bdrv_get_allocated_file_size
bdrv_poll_co <- Waiting with qemu_global_mutex locked

[coroutine] bdrv_co_get_allocated_file_size_entry
bdrv_co_get_allocated_file_size
raw_co_get_allocated_file_size
fstat <- SLOW!

The closest I got was moving the coroutine into a separate iothread,
unlocking the global mutex and releasing the bdrv aio_context around
aio_poll. It feels wrong though because we're technically still
operating on the block state but not holding the context.

Is there a more standard way if doing this? Is it possible at all?

Thanks