On Tue, Oct 31, 2023 at 10:05:56AM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé <berra...@redhat.com> writes: > > > 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. > > > > Sure, I don't disagree. However up until last week we had a seemingly > usable "add-fd" command that allows the user to provide a *set of file > descriptors* to QEMU. It's just now that we're learning that interface > serves only a special use-case.
AFAICT though we don't need add-fd to support passing many files for our needs. Saving only requires a single FD. All others can be opened by dup(), so the limitation of add-fd is irrelevant surely ? > > > 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. > > Ok, so I think the way to go here is for QEMU to receive a file + offset > instead of an FD. That way QEMU can have adequate control of the > resources for the migration. I don't remember why we went on the FD > tangent. Is it not acceptable for libvirt to provide the file name + > offset? FD passing means QEMU does not need privileges to open the file which could be useful. 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 :|
