Hi all, > -----Original Message----- > From: H.J. Lu [mailto:hjl.to...@gmail.com] > Sent: Friday, January 12, 2018 7:36 PM > To: Martin Jambor <mjam...@suse.cz> > Cc: Nagarajan, Muthu kumar raj <muthukumarraj.nagara...@amd.com>; > Kumar, Venkataramanan <venkataramanan.ku...@amd.com>; GCC > Patches <gcc-patches@gcc.gnu.org>; Jeff Law <l...@redhat.com> > Subject: Re: [PATCH 2/5] x86: Add -mindirect-branch-loop= > > 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. >
As per AMD architects, using “lfence” in “retpoline” is better than “pause” for our targets. So please allow filler to use "lfence". Regards. Venkat. > -- > H.J.
