On 01/12/2018 07:05 AM, H.J. Lu wrote: > On Fri, Jan 12, 2018 at 4:38 AM, Martin Jambor <mjam...@suse.cz> wrote: >> Hi, >> >> On Thu, Jan 11 2018, Jeff Law wrote: >>> On 01/07/2018 03:59 PM, H.J. Lu wrote: >>>> Add -mindirect-branch-loop= option to control loop filler in call and >>>> return thunks generated by -mindirect-branch=. 'lfence' uses "lfence" >>>> as loop filler. 'pause' uses "pause" as loop filler. 'nop' uses "nop" >>>> as loop filler. The default is 'lfence'. >>>> >>>> gcc/ >>>> >>>> * config/i386/i386-opts.h (indirect_branch_loop): New. >>>> * config/i386/i386.c (output_indirect_thunk): Support >>>> -mindirect-branch-loop=. >>>> * config/i386/i386.opt (mindirect-branch-loop=): New option. >>>> (indirect_branch_loop): New. >>>> (lfence): Likewise. >>>> (pause): Likewise. >>>> (nop): Likewise. >>>> * doc/invoke.texi: Document -mindirect-branch-loop= option. >>>> >>>> gcc/testsuite/ >>>> >>>> * gcc.target/i386/indirect-thunk-loop-1.c: New test. >>>> * gcc.target/i386/indirect-thunk-loop-2.c: Likewise. >>>> * gcc.target/i386/indirect-thunk-loop-3.c: Likewise. >>>> * gcc.target/i386/indirect-thunk-loop-4.c: Likewise. >>>> * gcc.target/i386/indirect-thunk-loop-5.c: Likewise. >>> I think we should drop the ability to change the filler until such time >>> as we really need it. Just pick one and go with it. I think David >>> suggested that they wanted "pause". I'm obviously fine with that. >>> >> >> unless I am mistaken (which is frankly quite possible, I am still not >> quite up to speed about the nuances), AMD strongly prefers the lfence >> variant. OTOH, IIUC, in kernel this will be run-time patched but so it >> does not matter in the most pressing case and we might want to have a >> mechanism doing something similar for protecting userspace later on. >> But perhaps it is enough to keep the option? >> >> Muthu and/or Venkat, can you please comment? > > If we do want it, I will submit a separate patch AFTER the current patch > set has been approved and checked into GCC 8. That would be my preference, if we need it at all.
jeff
