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

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Date:   Wed, 14 Apr 2021 18:10:33 +0200
From:   Eric Dumazet <edumazet@...gle.com>
To:     David Laight <David.Laight@...lab.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()

On Wed, Apr 14, 2021 at 6:08 PM David Laight <David.Laight@...lab.com> wrote:
>
> 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.
>

Yep, probably. I am using some recent clang version.

>         David
>
> -
> Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
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