On Mon, Oct 30, 2023 at 07:51:34PM -0300, Fabiano Rosas wrote: > I could use some advice on how to solve this situation. The fdset code > at monitor/fds.c and the add-fd command don't seem to be usable outside > the original use-case of passing fds with different open flags. > > There are several problems, the biggest one being that there's no way to > manipulate the set of file descriptors aside from asking for duplication > of an fd that matches a particular set of flags. > > That doesn't work for us because the two fds we need (one for main > channel, other for secondary channels) will have the same open flags. So > the fdset code will always return the first one it finds in the set.
QEMU may want multiple FDs *internally*, but IMHO that fact should not be exposed to mgmt applications. It would be valid for a QEMU impl to share the same FD across multiple threads, or have a different FD for each thread. All threads are using pread/pwrite, so it is safe for them to use the same FD if they desire. It is a private impl choice for QEMU at any given point in time and could change over time. Thus from the POV of the mgmt app, QEMU is writing to a single file, no matter how many threads are involved & thus it should only need to supply a single FD for thta file. QEMU can then call 'dup()' on that FD as many times as it desires, and use fcntl() to toggle O_DIRECT if and when it needs to. > Another problem (or feature) of the fdset code is that it only removes > an fd when qmp_remove_fd() is called if the VM runstate is RUNNING, > which means that the migration code cannot remove the fds after use > reliably. We need to be able to remove them to make sure we use the > correct fds in a subsequent migration. The "easy" option is to just add a new API that does what you want. Maybe during review someone will then point out why the orgianl API works the way it does. With regards, Daniel -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
