linux-kernel - RE: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq

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Message-ID: <1e5576a1a5b24cb0b1d53b9bb22d528e@AcuMS.aculab.com>
Date:   Wed, 14 Apr 2021 16:08:46 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Eric Dumazet' <edumazet@...gle.com>
CC:     Arjun Roy <arjunroy@...gle.com>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Eric Dumazet <eric.dumazet@...il.com>,
        "Ingo Molnar" <mingo@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        paulmck <paulmck@...nel.org>, Boqun Feng <boqun.feng@...il.com>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and
 clear_rseq_cs()

From: Eric Dumazet
> Sent: 14 April 2021 17:00
...
> > Repeated unsafe_get_user() calls are crying out for an optimisation.
> > You get something like:
> >         failed = 0;
> >         copy();
> >         if (failed) goto error;
> >         copy();
> >         if (failed) goto error;
> > Where 'failed' is set by the fault handler.
> >
> > This could be optimised to:
> >         failed = 0;
> >         copy();
> >         copy();
> >         if (failed) goto error;
> > Even if it faults on every invalid address it probably
> > doesn't matter - no one cares about that path.
> 
> 
> On which arch are you looking at ?
> 
> On x86_64 at least, code generation is just perfect.
> Not even a conditional jmp, it is all handled by exceptions (if any)
> 
> stac
> copy();
> copy();
> clac
> 
> 
> <out_of_line>
> efault_end: do error recovery.

It will be x86_64.
I'm definitely seeing repeated tests of (IIRC) %rdx.

It may well be because the compiler isn't very new.
Will be an Ubuntu build of 9.3.0.
Does that support 'asm goto with outputs' - which
may be the difference.

	David

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