linux-kernel - Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq

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Date:   Wed, 15 Jul 2020 09:31:24 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Florian Weimer <fweimer@...hat.com>
Cc:     carlos <carlos@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        paulmck <paulmck@...ux.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        "H. Peter Anvin" <hpa@...or.com>, Paul Turner <pjt@...gle.com>,
        linux-api <linux-api@...r.kernel.org>,
        Christian Brauner <christian.brauner@...ntu.com>
Subject: Re: [RFC PATCH 2/4] rseq: Allow extending struct rseq

----- On Jul 15, 2020, at 9:22 AM, Florian Weimer fweimer@...hat.com wrote:

> * Mathieu Desnoyers:
> 
>> Practically speaking, I suspect this would mean postponing availability of
>> rseq for widely deployed applications for a few more years ?
> 
> There is no rseq support in GCC today, so you have to write assembler
> code anyway.

Assembler code is only needed for the rseq critical sections, not for accessing
the __rseq_abi. Use-cases like getting the current cpu number do not currently
require any assembler for instance.

So indeed it could be done today without upgrading the toolchains by writing
custom assembler for each architecture to get the thread's struct rseq. AFAIU
the ABI to access the thread pointer is fixed for each architecture, right ?

How would this allow early-rseq-adopter libraries to interact with glibc ?

Thanks,

Mathieu

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