RFC: x86: Enable -z separate-code by default

[Michael Matz]

Hi,

On Thu, 22 Feb 2018, H.J. Lu wrote:

> >> Meh :-( Have you done any performance measurements on, say, SPECcpu?
> >
> > We will.
> 
> We compared SPEC CPU 2017 performance before and after this change on 
> Skylake server.  There are no any significant performance changes. 
> Everything is mostly below +/-1%.

Thanks, seems fine.

> >>> Should I add an ld configure option, --enable-separate-code, instead?
> >>
> >> Yes, definitely.  For old distros at least I definitely want to retain old
> >> behaviour by default and a configure switch would make that easier.
> >
> > Will do.
> >
> 
> Here is the patch to add --enable-separate-code to ld configure and
> enable it by default for Linux/x86.  But the default maximum page size
> is changed to 4KB for x86-64.
> 
> Any comments?

Should the max page size setting (without override on cmdline) depend on 
separate-code, or not?  I'm split minds here, I'm worried that the 2mb 
page size optimization we had for x86-64 isn't effective by default, but 
OTOH it already isn't since relro, so can't be that bad either, plus the 
kernel is of course free to use 2mb TLB entries when in the end the 
moons^Waddresses happen to be aligned right.


Ciao,
Michael.