On Thu, May 23, 2024 at 04:05:43PM -0300, Fabiano Rosas wrote: > We're about to enable the use of O_DIRECT in the migration code and > due to the alignment restrictions imposed by filesystems we need to > make sure the flag is only used when doing aligned IO. > > The migration will do parallel IO to different regions of a file, so > we need to use more than one file descriptor. Those cannot be obtained > by duplicating (dup()) since duplicated file descriptors share the > file status flags, including O_DIRECT. If one migration channel does > unaligned IO while another sets O_DIRECT to do aligned IO, the > filesystem would fail the unaligned operation. > > The add-fd QMP command along with the fdset code are specifically > designed to allow the user to pass a set of file descriptors with > different access flags into QEMU to be later fetched by code that > needs to alternate between those flags when doing IO. > > Extend the fdset matching to behave the same with the O_DIRECT flag. > > Signed-off-by: Fabiano Rosas <faro...@suse.de>
Reviewed-by: Peter Xu <pet...@redhat.com> One thing I am confused but totally irrelevant to this specific change. I wonder why do we need dupfds at all, and why client needs to remove-fd at all. It's about what would go wrong if qmp client only add-fd, then if it's consumed by anything, it gets moved from "fds" list to "dupfds" list. The thing is I don't expect the client should pass over any fd that will not be consumed. Then if it's always consumed, why bother dup() at all, and why bother an explicit remove-fd. -- Peter Xu
