From: Markus Armbruster <armbru@redhat.com>
To: Eric Blake <eblake@redhat.com>
Cc: "QEMU Trivial" <qemu-trivial@nongnu.org>,
"Peter Maydell" <peter.maydell@linaro.org>,
"Daniel P. Berrangé" <berrange@redhat.com>,
"QEMU Developers" <qemu-devel@nongnu.org>
Subject: Re: [PATCH] osdep.h: Add doc comment for qemu_get_thread_id()
Date: Fri, 31 Jul 2020 09:44:37 +0200 [thread overview]
Message-ID: <87pn8ct9ga.fsf@dusky.pond.sub.org> (raw)
In-Reply-To: <4d2cba04-04d8-9b82-562f-acb84b6010d2@redhat.com> (Eric Blake's message of "Thu, 30 Jul 2020 11:24:51 -0500")
Eric Blake <eblake@redhat.com> writes:
> 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).
Options for systems where don't know how to compute a system-wide thread
ID:
0. Return a bogus value: the PID. This is the status quo.
1. Return a more obviously bogus value: -1. Semantic compatibility
break. Should be harmless, because a QMP client relying on the
thread-id being the PID would be insane.
2. Make thread-id optional, present iff we can compute a value.
This is what we should have done, but we didn't, and now it's a
syntactic compatibility break. Matters only if it actually breaks
QMP clients. We believe the one we know shouldn't break.
Preferences?
next prev parent reply other threads:[~2020-07-31 7:45 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-07-16 15:41 [PATCH] osdep.h: Add doc comment for qemu_get_thread_id() Peter Maydell
2020-07-28 15:02 ` Peter Maydell
2020-07-28 15:16 ` Eric Blake
2020-07-28 15:25 ` Peter Maydell
2020-07-30 15:10 ` Markus Armbruster
2020-07-30 15:52 ` Peter Maydell
2020-07-30 15:59 ` Daniel P. Berrangé
2020-07-30 16:24 ` Eric Blake
2020-07-30 16:40 ` Daniel P. Berrangé
2020-07-31 7:44 ` Markus Armbruster [this message]
2020-07-31 13:46 ` Eric Blake
2020-07-31 13:51 ` Daniel P. Berrangé
2020-10-12 15:33 [PATCH 00/10] target/arm: Various v8.1M minor features Peter Maydell
2020-10-12 15:33 ` [PATCH] osdep.h: Add doc comment for qemu_get_thread_id() Peter Maydell
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