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Date:   Wed, 15 Jul 2020 10:38:22 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Florian Weimer <fw@deneb.enyo.de>
Cc:     Chris Kennelly <ckennelly@google.com>,
        Peter Oskolkov <posk@google.com>,
        Peter Oskolkov <posk@posk.io>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        paulmck <paulmck@linux.ibm.com>,
        Boqun Feng <boqun.feng@gmail.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Paul Turner <pjt@google.com>,
        linux-api <linux-api@vger.kernel.org>,
        Christian Brauner <christian.brauner@ubuntu.com>,
        carlos <carlos@redhat.com>
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Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq
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Thread-Topic: rseq: Allow extending struct rseq
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----- On Jul 15, 2020, at 2:31 AM, Florian Weimer fw@deneb.enyo.de wrote:

> * Chris Kennelly:
> 
>> When glibc provides registration, is the anticipated use case that a
>> library would unregister and reregister each thread to "upgrade" it to
>> the most modern version of interface it knows about provided by the
>> kernel?
> 
> Absolutely not, that is likely to break other consumers because an
> expected rseq area becomes dormant instead.

Indeed.

> 
>> There, I could assume an all-or-nothing registration of the new
>> feature--limited only by kernel availability for thread
>> homogeneity--but inconsistencies across early adopter libraries would
>> mean each thread would have to examine its own TLS to determine if a
>> feature were available.
> 
> Exactly.  Certain uses of seccomp can also have this effect,
> presenting a non-homogeneous view.

The nice thing about having a consistent feature-set for a given
thread group is that it allows specializing the code at thread
group startup, rather than requiring to dynamically check for
feature availability at runtime in fast-paths.

I wonder whether this kind of non-homogeneous view scenario
caused by seccomp is something we should support, or something
that should be documented as incompatible with rseq ?

Thanks,

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com