Hi HJ, > -----Original Message----- > From: Kumar, Venkataramanan > Sent: Friday, January 12, 2018 8:39 PM > To: 'H.J. Lu' <hjl.to...@gmail.com>; 'Martin Jambor' <mjam...@suse.cz> > Cc: Nagarajan, Muthu kumar raj <muthukumarraj.nagara...@amd.com>; > 'GCC Patches' <gcc-patches@gcc.gnu.org>; 'Jeff Law' <l...@redhat.com>; > Uros Bizjak (ubiz...@gmail.com) <ubiz...@gmail.com>; 'Jan Hubicka' > <j...@suse.de> > Subject: RE: [PATCH 2/5] x86: Add -mindirect-branch-loop= > > Hi all, > > > -----Original Message----- > > From: Kumar, Venkataramanan > > Sent: Friday, January 12, 2018 8:16 PM > > To: 'H.J. Lu' <hjl.to...@gmail.com>; Martin Jambor <mjam...@suse.cz> > > Cc: Nagarajan, Muthu kumar raj <muthukumarraj.nagara...@amd.com>; > > GCC Patches <gcc-patches@gcc.gnu.org>; Jeff Law <l...@redhat.com>; > Uros > > Bizjak (ubiz...@gmail.com) <ubiz...@gmail.com>; 'Jan Hubicka' > > <j...@suse.de> > > Subject: RE: [PATCH 2/5] x86: Add -mindirect-branch-loop= > > > > 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". > > > We also leant that "lfence" is a dispatch serializing instruction. The Puse > instruction is not serializing on AMD processors and has high latencies. > Any reason why Intel has chosen "pause" over "lfence" as the default loop filler for Retpoline?
> Regards. > Venkat. > > > -- > > > H.J.
