On Thu, Jul 30, 2020 at 04:52:49PM +0100, Peter Maydell wrote: > On Thu, 30 Jul 2020 at 16:11, Markus Armbruster <[email protected]> wrote: > > > > Peter Maydell <[email protected]> writes: > > > > > On Tue, 28 Jul 2020 at 16:17, Eric Blake <[email protected]> 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 <[email protected]> > > >> > --- > > >> > 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 -- |: https://berrange.com -o- https://www.flickr.com/photos/dberrange :| |: https://libvirt.org -o- https://fstop138.berrange.com :| |: https://entangle-photo.org -o- https://www.instagram.com/dberrange :|
