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 10:15:07 -0700
From:   Arjun Roy <arjunroy@...gle.com>
To:     Eric Dumazet <edumazet@...gle.com>
Cc:     David Laight <David.Laight@...lab.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 9:10 AM Eric Dumazet <edumazet@...gle.com> wrote:
>
> 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.
>

On x86-64 I can confirm, for me it (4 x unsafe_get_user()) compiles
down to stac + lfence + 8 x mov + clac as straight line code. And
results in roughly a 5%-10% speedup over copy_from_user().

-Arjun


> >         David
> >
> > -
> > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
> > Registration No: 1397386 (Wales)