Re: [PATCH v2 28/29] migration: Add direct-io parameter

["Daniel P. Berrangé" <berrange@redhat.com>]

On Tue, Oct 31, 2023 at 10:05:56AM -0300, Fabiano Rosas wrote:
> Daniel P. Berrangé <berrange@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
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