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

[Fabiano Rosas <farosas@suse.de>]

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.

> 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?

>> 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.

Hehe so I'll add a qmp_actually_remove_fd() =)