lkml.org 
[lkml]   [2021]   [Apr]   [14]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
From
Date
SubjectRe: [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@aculab.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
> Registration No: 1397386 (Wales)

\
 
 \ /
  Last update: 2021-04-14 18:11    [W:0.069 / U:1.520 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site