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? Thank you, Martin
