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RE: [PATCH 2/5] x86: Add -mindirect-branch-loop=
- From: "Kumar, Venkataramanan" <Venkataramanan dot Kumar at amd dot com>
- To: "Van De Ven, Arjan" <arjan dot van dot de dot ven at intel dot com>, David Woodhouse <dwmw2 at infradead dot org>, "H.J. Lu" <hjl dot tools at gmail dot com>, Jeff Law <law at redhat dot com>, Paul Turner <pjt at google dot com>, "Mallick, Asit K" <asit dot k dot mallick at intel dot com>
- Cc: "Nagarajan, Muthu kumar raj" <Muthukumarraj dot Nagarajan at amd dot com>, "GCC Patches" <gcc-patches at gcc dot gnu dot org>, Martin Jambor <mjambor at suse dot cz>, "Uros Bizjak (ubizjak at gmail dot com)" <ubizjak at gmail dot com>, Jan Hubicka <jh at suse dot de>, "Dharmakan, Rohit arul raj" <Rohitarulraj dot Dharmakan at amd dot com>
- Date: Sun, 14 Jan 2018 09:02:41 +0000
- Subject: RE: [PATCH 2/5] x86: Add -mindirect-branch-loop=
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Hi Arjan,
> -----Original Message-----
> From: Van De Ven, Arjan [mailto:arjan.van.de.ven@intel.com]
> Sent: Saturday, January 13, 2018 10:16 PM
> To: David Woodhouse <dwmw2@infradead.org>; Kumar, Venkataramanan
> <Venkataramanan.Kumar@amd.com>; H.J. Lu <hjl.tools@gmail.com>; Jeff
> Law <law@redhat.com>; Paul Turner <pjt@google.com>; Mallick, Asit K
> <asit.k.mallick@intel.com>
> Cc: Nagarajan, Muthu kumar raj <Muthukumarraj.Nagarajan@amd.com>;
> GCC Patches <gcc-patches@gcc.gnu.org>; Martin Jambor
> <mjambor@suse.cz>; Uros Bizjak (ubizjak@gmail.com)
> <ubizjak@gmail.com>; Jan Hubicka <jh@suse.de>; Dharmakan, Rohit arul raj
> <Rohitarulraj.Dharmakan@amd.com>
> Subject: RE: [PATCH 2/5] x86: Add -mindirect-branch-loop=
>
> > > If everyone is ok, my suggestion is to use "lfence" as the default
> > > loop filler for retpoline.
>
> can we do BOTH a pause and lfence.
> (that way on cpu's where pause is the power stop, it works, and on cpus
> where it's a fallthrough (AMD) it goes to the lfence)
>
I checked with our Architect. Having just "pause" is the concern.
It should also be fine for AMD to use "pause" followed by "lfence" in the loop of retpoline.
Regards,
Venkat.