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

From: Daniel P . Berrangé
Subject: Re: [PATCH] osdep.h: Add doc comment for qemu_get_thread_id()
Date: Thu, 30 Jul 2020 17:40:23 +0100
User-agent: Mutt/1.14.5 (2020-06-23)

On Thu, Jul 30, 2020 at 11:24:51AM -0500, Eric Blake wrote:
> On 7/30/20 10:59 AM, Daniel P. Berrangé wrote:
> > > 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.
> Normally, converting a mandatory output value to optional is a
> back-compatibility risk (we could break apps that depended on it being
> present).  But if the only apps that depended on it being present are
> compiled on Linux, where the member will actually be present, I think that
> changing the schema to make it optional for non-Linux platforms won't be a
> back-compatibility nightmare (but we will have to be careful in our
> documentation).

FWIW, libvirt treats it as mandatory for query-iothreads, but optional
for query-cpus because it was missing in some older QEMU versions

Libvirt explicitly only supports macOS, Linux and FreeBSD, so if those
platforms all report a value, libvirt won't care if you make it optional
and omit it for other platforms.

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