From: David Laight <David.Laight@ACULAB.COM>
To: 'Eric Dumazet' <edumazet@google.com>
Cc: Arjun Roy <arjunroy@google.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Eric Dumazet <eric.dumazet@gmail.com>,
"Ingo Molnar" <mingo@kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
paulmck <paulmck@kernel.org>, Boqun Feng <boqun.feng@gmail.com>,
linux-kernel <linux-kernel@vger.kernel.org>
Subject: RE: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs()
Date: Wed, 14 Apr 2021 16:08:46 +0000 [thread overview]
Message-ID: <1e5576a1a5b24cb0b1d53b9bb22d528e@AcuMS.aculab.com> (raw)
In-Reply-To: <CANn89iJj9254GORTsabwv6ZBPWzebR4iYAj9VYxuOZSrF99fNg@mail.gmail.com>
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
-
Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2021-04-14 16:08 UTC|newest]
Thread overview: 26+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-04-13 16:22 [PATCH v2 0/3] rseq: minor optimizations Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 1/3] rseq: optimize rseq_update_cpu_id() Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 2/3] rseq: remove redundant access_ok() Eric Dumazet
2021-04-13 16:22 ` [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs() Eric Dumazet
2021-04-13 16:54 ` Mathieu Desnoyers
2021-04-13 16:57 ` Eric Dumazet
2021-04-13 17:01 ` Eric Dumazet
2021-04-13 17:07 ` Eric Dumazet
2021-04-13 17:20 ` Mathieu Desnoyers
2021-04-13 17:33 ` Eric Dumazet
2021-04-13 18:00 ` Mathieu Desnoyers
2021-04-13 18:22 ` Eric Dumazet
2021-04-13 18:35 ` Arjun Roy
2021-04-13 21:19 ` David Laight
2021-04-13 22:03 ` Arjun Roy
2021-04-14 7:55 ` David Laight
2021-04-14 16:00 ` Eric Dumazet
2021-04-14 16:08 ` David Laight [this message]
2021-04-14 16:10 ` Eric Dumazet
2021-04-14 17:15 ` Arjun Roy
2021-04-14 17:35 ` Eric Dumazet
2021-04-14 20:15 ` Arjun Roy
2021-04-14 20:25 ` Eric Dumazet
2021-04-14 20:35 ` Arjun Roy
2021-04-13 19:13 ` Mathieu Desnoyers
2021-04-13 17:06 ` Mathieu Desnoyers
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