On Tue, Oct 31, 2023 at 11:33:24AM -0300, Fabiano Rosas wrote: > Daniel P. Berrangé <berra...@redhat.com> writes: > > > 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 ? > > Only once we decide to use one FD. If we had a generic add-fd backend, > then that's already a user-facing API, so the "implementation detail" > argument becomes weaker. > > With a single FD we'll need to be very careful about what code is > allowed to run while the multifd channels are doing IO. Since O_DIRECT > is not widely supported, now we have to also be careful about someone > using that QEMUFile handle to do unaligned writes and not even noticing > that it breaks direct IO. None of this in unworkable, of course, I just > find the design way clearer with just the file name + offset.
I guess I'm not seeing the problem still. A single FD is passed across from libvirt, but QEMU is free to turn that into *many* FDs for its internal use, using dup() and then setting O_DIRECT on as many/few of the dup()d FDs as its wants to. 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 :|
