On Fri, Jan 12, 2018 at 10:27 AM, Kumar, Venkataramanan <[email protected]> wrote: > Hi HJ, > >> -----Original Message----- >> From: Kumar, Venkataramanan >> Sent: Friday, January 12, 2018 8:39 PM >> To: 'H.J. Lu' <[email protected]>; 'Martin Jambor' <[email protected]> >> Cc: Nagarajan, Muthu kumar raj <[email protected]>; >> 'GCC Patches' <[email protected]>; 'Jeff Law' <[email protected]>; >> Uros Bizjak ([email protected]) <[email protected]>; 'Jan Hubicka' >> <[email protected]> >> 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' <[email protected]>; Martin Jambor <[email protected]> >> > Cc: Nagarajan, Muthu kumar raj <[email protected]>; >> > GCC Patches <[email protected]>; Jeff Law <[email protected]>; >> Uros >> > Bizjak ([email protected]) <[email protected]>; 'Jan Hubicka' >> > <[email protected]> >> > Subject: RE: [PATCH 2/5] x86: Add -mindirect-branch-loop= >> > >> > Hi all, >> > >> > > -----Original Message----- >> > > From: H.J. Lu [mailto:[email protected]] >> > > Sent: Friday, January 12, 2018 7:36 PM >> > > To: Martin Jambor <[email protected]> >> > > Cc: Nagarajan, Muthu kumar raj >> <[email protected]>; >> > > Kumar, Venkataramanan <[email protected]>; GCC >> > Patches >> > > <[email protected]>; Jeff Law <[email protected]> >> > > Subject: Re: [PATCH 2/5] x86: Add -mindirect-branch-loop= >> > > >> > > On Fri, Jan 12, 2018 at 4:38 AM, Martin Jambor <[email protected]> >> 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 -- H.J.
