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

[Daniel P . Berrangé]

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
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