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. -- H.J.
