Messages in this thread | | | From | Arjun Roy <> | Date | Wed, 14 Apr 2021 10:15:07 -0700 | Subject | Re: [PATCH v2 3/3] rseq: optimise rseq_get_rseq_cs() and clear_rseq_cs() |
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On Wed, Apr 14, 2021 at 9:10 AM Eric Dumazet <edumazet@google.com> wrote: > > 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. >
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)
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