Re: [PATCH] osdep.h: Add doc comment for qemu_get_thread_id()

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

On Thu, Jul 30, 2020 at 04:52:49PM +0100, Peter Maydell wrote:
> On Thu, 30 Jul 2020 at 16:11, Markus Armbruster <armbru@redhat.com> wrote:
> >
> > Peter Maydell <peter.maydell@linaro.org> writes:
> >
> > > On Tue, 28 Jul 2020 at 16:17, Eric Blake <eblake@redhat.com> wrote:
> > >>
> > >> On 7/16/20 10:41 AM, Peter Maydell wrote:
> > >> > Add a documentation comment for qemu_get_thread_id(): since this
> > >> > is rather host-OS-specific it's useful if people writing the
> > >> > implementation and people thinking of using the function know
> > >> > what the purpose and limitations are.
> > >> >
> > >> > Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
> > >> > ---
> > >> > Based on conversation with Dan on IRC, and prompted by the recent
> > >> > patch to add OpenBSD support.
> > >> >
> > >> > Q: should we document exactly what the thread-id value is for
> > >> > each host platform in the QMP documentation ? Somebody writing
> > >> > a management layer app should ideally not have to grovel through
> > >> > the application to figure out what they should do with the
> > >> > integer value they get back from query-cpus...
> > >> >
> > >> >   include/qemu/osdep.h | 14 ++++++++++++++
> > >> >   1 file changed, 14 insertions(+)
> > >>
> > >> Do we need a counterpart change...
> > >>
> > >> >
> > >> > diff --git a/include/qemu/osdep.h b/include/qemu/osdep.h
> > >> > index 4841b5c6b5f..8279f72e5ed 100644
> > >> > --- a/include/qemu/osdep.h
> > >> > +++ b/include/qemu/osdep.h
> > >> > @@ -515,6 +515,20 @@ bool qemu_has_ofd_lock(void);
> > >> >
> > >> >   bool qemu_write_pidfile(const char *pidfile, Error **errp);
> > >> >
> > >> > +/**
> > >> > + * qemu_get_thread_id: Return OS-specific ID of current thread
> > >> > + *
> > >> > + * This function returns an OS-specific identifier of the
> > >> > + * current thread. This will be used for the "thread-id" field in
> > >> > + * the response to the QMP query-cpus and query-iothreads commands.
> > >>
> > >> ...to the qapi definition of query-cpus and query-iothreads?
> > >
> > > Well, that was my question above. Currently the QAPI documentation
> > > says absolutely nothing about what the thread-id values mean
> > > for any host OS (beyond "ID of the underlying host thread"), which
> > > means that any management layer application needs to look in the
> > > implementation to find out what they actually are...
> >
> > ... which they will have to do to actually use it for the purpose we
> > have in mind, namely:
> >
> > >> > + * The intention is that a VM management layer application can then
> > >> > + * use it to tie specific QEMU vCPU and IO threads to specific host
> > >> > + * CPUs using whatever the host OS's CPU affinity setting API is.
> > >> > + * New implementations of this function for new host OSes should
> > >> > + * return the most sensible integer ID that works for that purpose.
> > >> > + *
> > >> > + * This function should not be used for anything else inside QEMU.
> > >> > + */
> >
> > Do they?
> 
> Well, I suspect that management-layer code currently has
> gone for "assume we're always running on Linux" and was
> written by people who knew they were getting a Linux tid...

Yes, on the libvirt side, the functionality that relies on thread_is is
only compiled on Linux. If someone wants to use it on other OS, they'll
have to provide an impl using their platforms equivalent of
sched_setaffinity and friends since none of this stuff is standardized
across OS.


> > The PID is quite unlikely to be "an OS-specific identifier of the
> > current thread".  Shouldn't we fail instead of lie when we don't know
> > how to compute the truth?
> 
> Yeah, I think the default codepath is pretty bogus too. Should
> the QMP functions have a mechanism for saying "we don't know
> a thread-id on this platform" ?

Thread_id should be optional and thus not filled in if we
can't provide a sensible value. Unfortunately we made it
mandatory in QMP.


Regards,
Daniel
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