Hi All, > -----Original Message----- > From: H.J. Lu [mailto:hjl.to...@gmail.com] > Sent: Saturday, January 13, 2018 1:11 AM > To: Kumar, Venkataramanan <venkataramanan.ku...@amd.com> > Cc: Nagarajan, Muthu kumar raj <muthukumarraj.nagara...@amd.com>; > GCC Patches <gcc-patches@gcc.gnu.org>; Martin Jambor > <mjam...@suse.cz>; Jeff Law <l...@redhat.com>; Uros Bizjak > (ubiz...@gmail.com) <ubiz...@gmail.com>; Jan Hubicka <j...@suse.de>; > Dharmakan, Rohit arul raj <rohitarulraj.dharma...@amd.com> > Subject: Re: [PATCH 2/5] x86: Add -mindirect-branch-loop= > > On Fri, Jan 12, 2018 at 10:27 AM, Kumar, Venkataramanan > <venkataramanan.ku...@amd.com> wrote: > > 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? > > > > My original patch uses "lfence". I was asked to use "pause": > > https://gcc.gnu.org/ml/gcc-patches/2018-01/msg00969.html
If everyone is ok, my suggestion is to use "lfence" as the default loop filler for retpoline. Please confirm. > > -- > H.J.
