On Wed, Apr 14, 2021 at 10:35 AM Eric Dumazet <eduma...@google.com> wrote: > > On Wed, Apr 14, 2021 at 7:15 PM Arjun Roy <arjun...@google.com> wrote: > > > > On Wed, Apr 14, 2021 at 9:10 AM Eric Dumazet <eduma...@google.com> wrote: > > > > > > On Wed, Apr 14, 2021 at 6:08 PM David Laight <david.lai...@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(). > > > > But rseq_get_rseq_cs() would still need three different copies, > with 3 stac+lfence+clac sequences. > > Maybe we need to enclose all __rseq_handle_notify_resume() operations > in a single section. > >
To provide a bit of further exposition on this point, if you do 4x unsafe_get_user() recall I mentioned a 5-10% improvement. On the other hand, 4x normal get_user() I saw something like a 100% (ie. doubling of sys time measured) regression. I assume that's the fault of multiple stac+clac. -Arjun > > > > > > > -Arjun > > > > > > > > David > > > > > > > > - > > > > Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, > > > > MK1 1PT, UK > > > > Registration No: 1397386 (Wales)
