Daniel P. Berrangé <berra...@redhat.com> writes: > 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.
The problem is that duplicated FDs share the file status flags. If we set O_DIRECT on the multifd channels and the main thread happens to do an unaligned write with qemu_file_put* then the filesystem will fail that write.
