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

[Peter Maydell]

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...

> 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" ?

thanks
-- PMM