linux-kernel - Re: [RFC PATCH for 4.15 v12 00/22] Restartable sequences and CPU op vector

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Date:   Wed, 22 Nov 2017 20:32:19 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Cc:     Andi Kleen <andi@...stfloor.org>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Andy Lutomirski <luto@...capital.net>,
        Dave Watson <davejwatson@...com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King <linux@....linux.org.uk>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andrew Hunter <ahh@...gle.com>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>
Subject: Re: [RFC PATCH for 4.15 v12 00/22] Restartable sequences and CPU op
 vector

On Tue, Nov 21, 2017 at 10:05:08PM +0000, Mathieu Desnoyers wrote:
> Other than that, I have not received any concrete alternative proposal to
> properly handle single-stepping.

That's not entirely true; amluto did have an alternative in Prague: do
full machine level instruction emulation till the end of the rseq when
it gets 'preempted too often'.

Yes, implementing that will be an absolute royal pain. But it does
remove the whole duplicate/dual program asm/bytecode thing and avoids
the syscall entirely.

And we don't need to do a full x86_64/arch-of-choice emulator for this
either; just as cpu_opv is fairly limited too. We can do a subset that
allows dealing with the known sequences and go from there -- it can
always fall back to not emulating and reverting to the pure rseq with
debug/fwd progress 'issues'.

So what exactly is the problem of leaving out the whole cpu_opv thing
for now? Pure rseq is usable -- albeit a bit cumbersome without
additional debugger support.