On Tue, Dec 15, 2015 at 1:06 PM, Carlos O'Donell <carlos@xxxxxxxxxx> wrote: > On 12/15/2015 03:08 PM, H.J. Lu wrote: >> On Tue, Dec 15, 2015 at 10:38 AM, Carlos O'Donell <carlos@xxxxxxxxxx> wrote: >>> > On 12/15/2015 01:27 PM, Carlos O'Donell wrote: >>>> >> + cpu_features->feature[index_Prefer_MAP_32BIT_EXEC] >>>> >> + |= get_prefer_map_32bit_exec (); >>>> >> >>>> >> You wouldn't need get_prefer_map_32bit_exec, since this is all part of >>>> >> the code, like dl-librecon.h, which parses the extra env var. >>> > >>> > To be clear: >>> > >>> > * Add new bit flag definitions for cpu_features. >>> > * Add a sysdeps/unix/sysv/linux/x86_64/dl-silvermont.h >>> > * Fill in EXTRA_LD_ENVVARS or add new ones. >>> > * Write to rtld's GLRO cpu_features the bit you need based >>> > on __libc_enable_secure. >>> > >>> > That should simplify and concentrate the Silvermont fixes to >>> > just two files, making future maintenance and documentation >>> > easier. >>> > >>> > >> This is the updated patch. I put EXTRA_LD_ENVVARS and >> EXTRA_UNSECURE_ENVVARS in x86_64/64/dl-librecon.h >> to be consistent with i386/dl-librecon.h. If we ever need to >> update EXTRA_LD_ENVVARS/EXTRA_UNSECURE_ENVVARS >> in the future, we have a single file to change. >> >> Tested on x86-64. OK for master? >> >> Thanks for all the feedbacks and suggestions. > > This looks much better and much cleaner. Looks good to me now. It also > appears you have consesnsus with this last change. > > It needs a bug # please since you're fixing a user-visible performance > problem on Silvermont. I opened https://sourceware.org/bugzilla/show_bug.cgi?id=19367 and checked in my patch. > It appears to meet Zack's requirement to choose a security safe default > at the expense of performance (I agree with that). > > I *strongly* urge you to immediately submit a patch to the linux man > pages project to document that as of 2.23 this new flag exists and > does what you describe it does. > Here is a patch for Linux man page. Thanks. H.J.
From 6b8095fb2e123915ec2e586753d3d7ff7f14aee1 Mon Sep 17 00:00:00 2001 From: "H.J. Lu" <hjl.tools@xxxxxxxxx> Date: Tue, 15 Dec 2015 13:29:37 -0800 Subject: [PATCH] Document LD_PREFER_MAP_32BIT_EXEC for ld.so (8) Glibc 2.23 added LD_PREFER_MAP_32BIT_EXEC to improve branch prediction performance on Silvermont. Signed-off-by: H.J. Lu <hjl.tools@xxxxxxxxx> --- man8/ld.so.8 | 10 ++++++++++ 1 file changed, 10 insertions(+) diff --git a/man8/ld.so.8 b/man8/ld.so.8 index 608cf7c..da33404 100644 --- a/man8/ld.so.8 +++ b/man8/ld.so.8 @@ -472,6 +472,16 @@ environment variable has been set. (ELF only)(glibc since 2.1.3) If set to a nonempty string, warn about unresolved symbols. .TP +.B LD_PREFER_MAP_32BIT_EXEC +(x86-64 only)(glibc since 2.23) +According to Silvermont software optimization guide, for 64-bit +applications, branch prediction performance can be negatively impacted +when the target of a branch is more than 4GB away from the branch. +If set, ld.so will try to map executable pages with MAP_32BIT first. +NB: MAP_32BIT will map to lower 2GB, not lower 4GB, address. Since +MAP_32BIT reduces bits available for address space layout randomization +(ASLR), LD_PREFER_MAP_32BIT_EXEC is always disabled for SUID programs. +.TP .B LDD_ARGV0 (libc5) .IR argv [0] -- 2.5.0
